src/x509/asn1.t0

\ Copyright (c) 2016 Thomas Pornin <pornin@bolet.org>
\
\ Permission is hereby granted, free of charge, to any person obtaining
\ a copy of this software and associated documentation files (the
\ "Software"), to deal in the Software without restriction, including
\ without limitation the rights to use, copy, modify, merge, publish,
\ distribute, sublicense, and/or sell copies of the Software, and to
\ permit persons to whom the Software is furnished to do so, subject to
\ the following conditions:
\
\ The above copyright notice and this permission notice shall be
\ included in all copies or substantial portions of the Software.
\
\ THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
\ EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
\ MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
\ NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
\ BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
\ ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
\ CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
\ SOFTWARE.

\ =======================================================================

\ This file contains code which is common to all engines that do some
\ ASN.1 decoding. It should not be compiled on its own, but only along
\ with another file (e.g. x509_minimal.t0) which uses it.
\
\ Users must define several things:
\
\ -- In the preamble, a macro called "CTX" that evaluates to the current
\ context structure.
\
\ -- In the preamble, a macro called "CONTEXT_NAME" that evaluates to the
\ context structure type. This will be invoked during compilation.
\
\ -- A word called "read8-low" ( -- x ) that reads the next byte, or -1
\ if the input buffer is empty. That word is usually written in C.
\
\ -- A word called "read-blob-inner" ( addr len -- addr len ) that is
\ the multi-byte version of read8-low.
\
\ -- A word called "skip-remaining-inner" ( lim -- lim ) which reads but
\ drops some input bytes.

preamble {

#include "inner.h"

}

\ Read next source character, skipping blanks.
: skip-blanks begin char dup 32 > if ret then drop again ;

: fail-oid
	"Invalid OID" puts cr exitvm ;

\ Read a decimal integer, followed by either a dot or whitespace.
\ Note: this does not check for overflows.
: parse-number ( -- val nextchar )
	char decval
	begin
		char
		dup dup `. = swap 32 <= or if ret then
		decval swap 10 * +
	again ;

\ Encode a number in unsigned 7E format.
: encode7E ( val -- )
	0 encode7E-inner ;

: encode7E-inner ( val eb -- )
	swap dup 0x7F > if
		dup 7 u>> 0x80 encode7E-inner 0x7F and
	then
	or data-add8 ;

\ Decode an OID from source, and encode it. First byte is length,
\ followed by encoded ASN.1 DER value. The OID is encoded in the
\ current data block.
: OID
	\ Get current data address, and push a 0 for length.
	current-data 0 data-add8
	\ Skip blanks and get first digit, which must be 0, 1 or 2.
	skip-blanks decval dup 2 > if fail-oid then
	40 *
	\ Next character must be a dot.
	char `. <> if fail-oid then
	\ Second group must be one or two digits.
	parse-number { nextchar }
	dup 40 >= if fail-oid then
	+ encode7E
	\ While next character is a dot, keep encoding numbers.
	begin nextchar `. = while
		parse-number >nextchar
		encode7E
	repeat
	\ Write back length in the first byte.
	dup current-data swap - 1- swap data-set8
	; immediate

\ Define a new data word for an encoded OID. The OID is read from the
\ source.
: OID:
	new-data-block next-word define-data-word postpone OID ;

\ Define a word that evaluates to the address of a field within the
\ context.
: addr:
	next-word { field }
	"addr-" field + 0 1 define-word
	0 8191 "offsetof(CONTEXT_NAME, " field + ")" + make-CX
	postpone literal postpone ; ;

addr: pad

\ Define a word that evaluates to an error code through a macro name.
: err:
	next-word { name }
	name 0 1 define-word
	0 63 "BR_" name + make-CX postpone literal postpone ; ;

err: ERR_X509_INVALID_VALUE
err: ERR_X509_TRUNCATED
err: ERR_X509_EMPTY_CHAIN
err: ERR_X509_INNER_TRUNC
err: ERR_X509_BAD_TAG_CLASS
err: ERR_X509_BAD_TAG_VALUE
err: ERR_X509_INDEFINITE_LENGTH
err: ERR_X509_EXTRA_ELEMENT
err: ERR_X509_UNEXPECTED
err: ERR_X509_NOT_CONSTRUCTED
err: ERR_X509_NOT_PRIMITIVE
err: ERR_X509_PARTIAL_BYTE
err: ERR_X509_BAD_BOOLEAN
err: ERR_X509_OVERFLOW
err: ERR_X509_BAD_DN
err: ERR_X509_BAD_TIME
err: ERR_X509_UNSUPPORTED
err: ERR_X509_LIMIT_EXCEEDED
err: ERR_X509_WRONG_KEY_TYPE
err: ERR_X509_BAD_SIGNATURE
err: ERR_X509_EXPIRED
err: ERR_X509_DN_MISMATCH
err: ERR_X509_BAD_SERVER_NAME
err: ERR_X509_CRITICAL_EXTENSION
err: ERR_X509_NOT_CA
err: ERR_X509_FORBIDDEN_KEY_USAGE
err: ERR_X509_WEAK_PUBLIC_KEY

: KEYTYPE_RSA     CX 0 15 { BR_KEYTYPE_RSA } ;
: KEYTYPE_EC      CX 0 15 { BR_KEYTYPE_EC } ;

cc: fail ( err -- ! ) {
	CTX->err = T0_POPi();
	T0_CO();
}

\ Read one byte from the stream.
: read8-nc ( -- x )
	begin
		read8-low dup 0 >= if ret then
		drop co
	again ;

\ Read one byte, enforcing current read limit.
: read8 ( lim -- lim x )
	dup ifnot ERR_X509_INNER_TRUNC fail then
	1- read8-nc ;

\ Read a 16-bit value, big-endian encoding.
: read16be ( lim -- lim x )
	read8 8 << swap read8 rot + ;

\ Read a 16-bit value, little-endian encoding.
: read16le ( lim -- lim x )
	read8 swap read8 8 << rot + ;

\ Read all bytes from the current element, then close it (i.e. drop the
\ limit). Destination address is an offset within the context.
: read-blob ( lim addr -- )
	swap
	begin dup while read-blob-inner dup if co then repeat
	2drop ;

\ Skip remaining bytes in the current structure, but do not close it
\ (thus, this leaves the value 0 on the stack).
: skip-remaining ( lim -- lim )
	begin dup while skip-remaining-inner dup if co then repeat ;

: skip-remaining-inner ( lim -- lim )
	0 over read-blob-inner -rot 2drop ;

cc: set8 ( val addr -- ) {
	uint32_t addr = T0_POP();
	*((unsigned char *)CTX + addr) = (unsigned char)T0_POP();
}

cc: set16 ( val addr -- ) {
	uint32_t addr = T0_POP();
	*(uint16_t *)(void *)((unsigned char *)CTX + addr) = T0_POP();
}

cc: set32 ( val addr -- ) {
	uint32_t addr = T0_POP();
	*(uint32_t *)(void *)((unsigned char *)CTX + addr) = T0_POP();
}

cc: get8 ( addr -- val ) {
	uint32_t addr = T0_POP();
	T0_PUSH(*((unsigned char *)CTX + addr));
}

cc: get16 ( addr -- val ) {
	uint32_t addr = T0_POP();
	T0_PUSH(*(uint16_t *)(void *)((unsigned char *)CTX + addr));
}

cc: get32 ( addr -- val ) {
	uint32_t addr = T0_POP();
	T0_PUSH(*(uint32_t *)(void *)((unsigned char *)CTX + addr));
}

\ Read an ASN.1 tag. This function returns the "constructed" status
\ and the tag value. The constructed status is a boolean (-1 for
\ constructed, 0 for primitive). The tag value is either 0 to 31 for
\ a universal tag, or 32+x for a contextual tag of value x. Tag classes
\ "application" and "private" are rejected. Universal tags beyond 30
\ are rejected. Contextual tags beyond 30 are rejected. Thus, accepted
\ tags will necessarily fit on exactly one byte. This does not support
\ the whole of ASN.1/BER, but is sufficient for certificate parsing.
: read-tag ( lim -- lim constructed value )
	read8 { fb }

	\ Constructed flag is bit 5.
	fb 5 >> 0x01 and neg

	\ Class is in bits 6 and 7. Accepted classes are 00 (universal)
	\ and 10 (context). We check that bit 6 is 0, and shift back
	\ bit 7 so that we get 0 (universal) or 32 (context).
	fb 6 >> dup 0x01 and if ERR_X509_BAD_TAG_CLASS fail then
	4 <<

	\ Tag value is in bits 0..4. If the value is 31, then this is
	\ an extended tag, encoded over subsequent bytes, and we do
	\ not support that.
	fb 0x1F and dup 0x1F = if ERR_X509_BAD_TAG_VALUE fail then
	+ ;

\ Read a tag, but only if not at the end of the current object. If there
\ is no room for another element (limit is zero), then this will push a
\ synthetic "no tag" value (primitive, with value -1).
: read-tag-or-end ( lim -- lim constructed value )
	dup ifnot 0 -1 ret then
	read-tag ;

\ Compare the read tag with the provided value. If equal, then the
\ element is skipped, and a new tag is read (or end of object).
: iftag-skip ( lim constructed value ref -- lim constructed value )
	over = if
		2drop
		read-length-open-elt skip-close-elt
		read-tag-or-end
	then ;

\ Read an ASN.1 length. This supports only definite lengths (theoretically,
\ certificates may use an indefinite length for the outer structure, using
\ DER only in the TBS, but this never happens in practice, except in a
\ single example certificate from 15 years ago that also fails to decode
\ properly for other reasons).
: read-length ( lim -- lim length )
	read8
	\ Lengths in 0x00..0x7F get encoded as a single byte.
	dup 0x80 < if ret then

	\ If the byte is 0x80 then this is an indefinite length, and we
	\ do not support that.
	0x80 - dup ifnot ERR_X509_INDEFINITE_LENGTH fail then

	\ Masking out bit 7, this yields the number of bytes over which
	\ the value is encoded. Since the total certificate length must
	\ fit over 3 bytes (this is a consequence of SSL/TLS message
	\ format), we can reject big lengths and keep the length in a
	\ single integer.
	{ n } 0
	begin n 0 > while n 1- >n
		dup 0x7FFFFF > if ERR_X509_INNER_TRUNC fail then
		8 << swap read8 rot +
	repeat ;

\ Open a sub-structure. This subtracts the length from the limit, and
\ pushes the length back as new limit.
: open-elt ( lim length -- lim_outer lim_inner )
	dup2 < if ERR_X509_INNER_TRUNC fail then
	dup { len } - len ;

\ Read a length and open the value as a sub-structure.
: read-length-open-elt ( lim -- lim_outer lim_inner )
	read-length open-elt ;

\ Close a sub-structure. This verifies that there is no remaining
\ element to read.
: close-elt ( lim -- )
	if ERR_X509_EXTRA_ELEMENT fail then ;

\ Skip remaining bytes in the current structure, then close it.
: skip-close-elt ( lim -- )
	skip-remaining drop ;

\ Read a length and then skip the value.
: read-length-skip ( lim -- lim )
	read-length-open-elt skip-close-elt ;

\ Check that a given tag is constructed and has the expected value.
: check-tag-constructed ( constructed value refvalue -- )
	= ifnot ERR_X509_UNEXPECTED fail then
	check-constructed ;

\ Check that the top value is true; report a "not constructed"
\ error otherwise.
: check-constructed ( constructed -- )
	ifnot ERR_X509_NOT_CONSTRUCTED fail then ;

\ Check that a given tag is primitive and has the expected value.
: check-tag-primitive ( constructed value refvalue -- )
	= ifnot ERR_X509_UNEXPECTED fail then
	check-primitive ;

\ Check that the top value is true; report a "not primitive"
\ error otherwise.
: check-primitive ( constructed -- )
	if ERR_X509_NOT_PRIMITIVE fail then ;

\ Check that the tag is for a constructed SEQUENCE.
: check-sequence ( constructed value -- )
	0x10 check-tag-constructed ;

\ Read a tag, check that it is for a constructed SEQUENCE, and open
\ it as a sub-element.
: read-sequence-open ( lim -- lim_outer lim_inner )
	read-tag check-sequence read-length-open-elt ;

\ Read the next element as a BIT STRING with no ignore bits, and open
\ it as a sub-element.
: read-bits-open ( lim -- lim_outer lim_inner )
	read-tag 0x03 check-tag-primitive
	read-length-open-elt
	read8 if ERR_X509_PARTIAL_BYTE fail then ;

OID: rsaEncryption               1.2.840.113549.1.1.1

OID: sha1WithRSAEncryption       1.2.840.113549.1.1.5
OID: sha224WithRSAEncryption     1.2.840.113549.1.1.14
OID: sha256WithRSAEncryption     1.2.840.113549.1.1.11
OID: sha384WithRSAEncryption     1.2.840.113549.1.1.12
OID: sha512WithRSAEncryption     1.2.840.113549.1.1.13

OID: id-sha1                     1.3.14.3.2.26
OID: id-sha224                   2.16.840.1.101.3.4.2.4
OID: id-sha256                   2.16.840.1.101.3.4.2.1
OID: id-sha384                   2.16.840.1.101.3.4.2.2
OID: id-sha512                   2.16.840.1.101.3.4.2.3

OID: id-ecPublicKey              1.2.840.10045.2.1

OID: ansix9p256r1                1.2.840.10045.3.1.7
OID: ansix9p384r1                1.3.132.0.34
OID: ansix9p521r1                1.3.132.0.35

OID: ecdsa-with-SHA1             1.2.840.10045.4.1
OID: ecdsa-with-SHA224           1.2.840.10045.4.3.1
OID: ecdsa-with-SHA256           1.2.840.10045.4.3.2
OID: ecdsa-with-SHA384           1.2.840.10045.4.3.3
OID: ecdsa-with-SHA512           1.2.840.10045.4.3.4

OID: id-at-commonName            2.5.4.3

\ Read a "small value". This assumes that the tag has just been read
\ and processed, but not the length. The first pad byte is set to the
\ value length; the encoded value itself follows. If the value length
\ exceeds 255 bytes, then a single 0 is written in the pad, and this
\ method returns false (0). Otherwise, it returns true (-1).
\ Either way, the element is fully read.
: read-small-value ( lim -- lim bool )
	read-length-open-elt
	dup 255 > if skip-close-elt 0 addr-pad set8 0 ret then
	dup addr-pad set8
	addr-pad 1+ read-blob
	-1 ;

\ Read an OID as a "small value" (tag, length and value). A boolean
\ value is returned, which is true (-1) if the OID value fits on the pad,
\ false (0) otherwise.
: read-OID ( lim -- lim bool )
	read-tag 0x06 check-tag-primitive read-small-value ;

\ Read a UTF-8 code point. On error, return 0. Reading a code point of
\ value 0 is considered to be an error.
: read-UTF8 ( lim -- lim val )
	read8
	choice
		dup 0x80 < uf ret enduf
		dup 0xC0 < uf drop 0 ret enduf
		dup 0xE0 < uf 0x1F and 1 read-UTF8-next 0x80 0x7FF enduf
		dup 0xF0 < uf 0x0F and 2 read-UTF8-next 0x800 0xFFFF enduf
		dup 0xF8 < uf 0x07 and 3 read-UTF8-next 0x10000 0x10FFFF enduf
		drop 0 ret
	endchoice
	between? ifnot drop 0 then
	;

\ Read n subsequent bytes to complete the provided first byte. The final
\ value is -1 on error, or the code point numerical value. The final
\ value is duplicated.
: read-UTF8-next ( lim val n -- lim val val )
	begin dup while
		-rot
		read-UTF8-chunk
		rot 1-
	repeat
	drop dup ;

\ Read one byte, that should be a trailing UTF-8 byte, and complement the
\ current value. On error, value is set to -1.
: read-UTF8-chunk ( lim val -- lim val )
	swap
	\ If we are at the end of the value, report an error but don't fail.
	dup ifnot 2drop 0 -1 ret then
	read8 rot
	dup 0< if swap drop ret then 6 <<
	swap dup 6 >> 2 <> if 2drop -1 ret then
	0x3F and + ;

: high-surrogate? ( x -- x bool )
	dup 0xD800 0xDBFF between? ;

: low-surrogate? ( x -- x bool )
	dup 0xDC00 0xDFFF between? ;

: assemble-surrogate-pair ( hi lim lo -- lim val )
	low-surrogate? ifnot rot 2drop 0 ret then
	rot 10 << + 0x35FDC00 - ;

\ Read a UTF-16 code point (big-endian). Returned value is 0 on error.
: read-UTF16BE ( lim -- lim val )
	read16be
	choice
		high-surrogate? uf
			swap dup ifnot 2drop 0 0 ret then
			read16be assemble-surrogate-pair
		enduf
		low-surrogate? uf
			drop 0
		enduf
	endchoice ;

\ Read a UTF-16 code point (little-endian). Returned value is 0 on error.
: read-UTF16LE ( lim -- lim val )
	read16le
	choice
		high-surrogate? uf
			swap dup ifnot 2drop 0 0 ret then
			read16le assemble-surrogate-pair
		enduf
		low-surrogate? uf
			drop 0
		enduf
	endchoice ;

\ Add byte to current pad value. Offset is updated, or set to 0 on error.
: pad-append ( off val -- off )
	over dup 0= swap 256 >= or if 2drop 0 ret then
	over addr-pad + set8 1+ ;

\ Add UTF-8 chunk byte to the pad. The 'nn' parameter is the shift count.
: pad-append-UTF8-chunk ( off val nn -- off )
	>> 0x3F and 0x80 or pad-append ;

\ Test whether a code point is invalid when encoding. This rejects the
\ 66 noncharacters, and also the surrogate range; this function does NOT
\ check that the value is in the 0..10FFFF range.
: valid-unicode? ( val -- bool )
	dup 0xFDD0 0xFDEF between? if drop 0 ret then
	dup 0xD800 0xDFFF between? if drop 0 ret then
	0xFFFF and 0xFFFE < ;

\ Encode a code point in UTF-8. Offset is in the pad; it is updated, or
\ set to 0 on error. Leading BOM are ignored.
: encode-UTF8 ( val off -- off )
	\ Skip leading BOM (U+FEFF when off is 1).
	dup2 1 = swap 0xFEFF = and if swap drop ret then

	swap dup { val }
	dup valid-unicode? ifnot 2drop 0 ret then
	choice
		dup 0x80 < uf pad-append enduf
		dup 0x800 < uf
			6 >> 0xC0 or pad-append
			val 0 pad-append-UTF8-chunk
		enduf
		dup 0xFFFF < uf
			12 >> 0xE0 or pad-append
			val 6 pad-append-UTF8-chunk
			val 0 pad-append-UTF8-chunk
		enduf
		18 >> 0xF0 or pad-append
		val 12 pad-append-UTF8-chunk
		val 6 pad-append-UTF8-chunk
		val 0 pad-append-UTF8-chunk
	endchoice ;

\ Read a string value into the pad; this function checks that the source
\ characters are UTF-8 and non-zero. The string length (in bytes) is
\ written in the first pad byte. Returned value is true (-1) on success,
\ false (0) on error.
: read-value-UTF8 ( lim -- lim bool )
	read-length-open-elt
	1 { off }
	begin dup while
		read-UTF8 dup ifnot drop skip-close-elt 0 ret then
		off encode-UTF8 >off
	repeat
	drop off dup ifnot ret then 1- addr-pad set8 -1 ;

\ Decode a UTF-16 string into the pad. The string is converted to UTF-8,
\ and the length is written in the first pad byte. A leading BOM is
\ honoured (big-endian is assumed if there is no BOM). A code point of
\ value 0 is an error. Returned value is true (-1) on success, false (0)
\ on error.
: read-value-UTF16 ( lim -- lim bool )
	read-length-open-elt
	dup ifnot addr-pad set8 -1 ret then
	1 { off }
	read-UTF16BE dup 0xFFFE = if
		\ Leading BOM, and indicates little-endian.
		drop
		begin dup while
			read-UTF16LE dup ifnot drop skip-close-elt 0 ret then
			off encode-UTF8 >off
		repeat
	else
		dup ifnot drop skip-close-elt 0 ret then
		\ Big-endian BOM, or no BOM.
		begin
			off encode-UTF8 >off
			dup while
			read-UTF16BE dup ifnot drop skip-close-elt 0 ret then
		repeat
	then
	drop off dup ifnot ret then 1- addr-pad set8 -1 ;

\ Decode a latin-1 string into the pad. The string is converted to UTF-8,
\ and the length is written in the first pad byte. A source byte of
\ value 0 is an error. Returned value is true (-1) on success, false (0)
\ on error.
: read-value-latin1 ( lim -- lim bool )
	read-length-open-elt
	1 { off }
	begin dup while
		read8 dup ifnot drop skip-close-elt 0 ret then
		off encode-UTF8 >off
	repeat
	drop off dup ifnot ret then 1- addr-pad set8 -1 ;

\ Read a value and interpret it as an INTEGER or ENUMERATED value. If
\ the integer value does not fit on an unsigned 32-bit value, an error
\ is reported. This function assumes that the tag has just been read
\ and processed, but not the length.
: read-small-int-value ( lim -- lim x )
	read-length-open-elt
	dup ifnot ERR_X509_OVERFLOW fail then
	read8 dup 0x80 >= if ERR_X509_OVERFLOW fail then
	{ x }
	begin dup while
		read8 x dup 0xFFFFFF >= if ERR_X509_OVERFLOW fail then
		8 << + >x
	repeat
	drop x ;

\ Compare the OID in the pad with an OID in the constant data block.
\ Returned value is -1 on equality, 0 otherwise.
cc: eqOID ( addrConst -- bool ) {
	const unsigned char *a2 = &t0_datablock[T0_POP()];
	const unsigned char *a1 = &CTX->pad[0];
	size_t len = a1[0];
	int x;
	if (len == a2[0]) {
		x = -(memcmp(a1 + 1, a2 + 1, len) == 0);
	} else {
		x = 0;
	}
	T0_PUSH((uint32_t)x);
}

\ Compare two blobs in the context. Returned value is -1 on equality, 0
\ otherwise.
cc: eqblob ( addr1 addr2 len -- bool ) {
	size_t len = T0_POP();
	const unsigned char *a2 = (const unsigned char *)CTX + T0_POP();
	const unsigned char *a1 = (const unsigned char *)CTX + T0_POP();
	T0_PUSHi(-(memcmp(a1, a2, len) == 0));
}

\ Check that a value is in a given range (inclusive).
: between? ( x min max -- bool )
	{ min max } dup min >= swap max <= and ;

\ Convert the provided byte value into a number in the 0..9 range,
\ assuming that it is an ASCII digit. A non-digit triggers an error
\ (a "bad time" error since this is used in date/time decoding).
: digit-dec ( char -- value )
	`0 - dup 0 9 between? ifnot ERR_X509_BAD_TIME fail then ;

\ Read two ASCII digits and return the value in the 0..99 range. An
\ error is reported if the characters are not ASCII digits.
: read-dec2 ( lim -- lim x )
	read8 digit-dec 10 * { x } read8 digit-dec x + ;

\ Read two ASCII digits and check that the value is in the provided
\ range (inclusive).
: read-dec2-range ( lim min max -- lim x )
	{ min max }
	read-dec2 dup min max between? ifnot ERR_X509_BAD_TIME fail then ;

\ Maximum days in a month and accumulated day count. Each
\ 16-bit value contains the month day count in its lower 5 bits. The first
\ 12 values are for a normal year, the other 12 for a leap year.
data: month-to-days
hexb| 001F 03FC 077F 0B5E 0F1F 12FE 16BF 1A9F 1E7E 223F 261E 29DF |
hexb| 001F 03FD 079F 0B7E 0F3F 131E 16DF 1ABF 1E9E 225F 263E 29FF |

\ Read a date (UTCTime or GeneralizedTime). The date value is converted
\ to a day count and a second count. The day count starts at 0 for
\ January 1st, 0 AD (that's they year before 1 AD, also known as 1 BC)
\ in a proleptic Gregorian calendar (i.e. Gregorian rules are assumed to
\ extend indefinitely in the past). The second count is between 0 and
\ 86400 (inclusive, in case of a leap second).
: read-date ( lim -- lim days seconds )
	\ Read tag; must be UTCTime or GeneralizedTime. Year count is
	\ 4 digits with GeneralizedTime, 2 digits with UTCTime.
	read-tag
	dup 0x17 0x18 between? ifnot ERR_X509_BAD_TIME fail then
	0x18 = { y4d }
	check-primitive
	read-length-open-elt

	\ We compute the days and seconds counts during decoding, in
	\ order to minimize the number of needed temporary variables.
	{ ; days seconds x }

	\ Year is 4-digit with GeneralizedTime. With UTCTime, the year
	\ is in the 1950..2049 range, and only the last two digits are
	\ present in the encoding.
	read-dec2
	y4d if
		100 * >x read-dec2 x +
	else
		dup 50 < if 100 + then 1900 +
	then
	>x
	x 365 * x 3 + 4 / + x 99 + 100 / - x 399 + 400 / + >days

	\ Month is 1..12. Number of days in a months depend on the
	\ month and on the year (year count is in x at that point).
	1 12 read-dec2-range
	1- 1 <<
	x 4 % 0= x 100 % 0<> x 400 % 0= or and if 24 + then
	month-to-days + data-get16
	dup 5 >> days + >days
	0x1F and

	\ Day. At this point, the TOS contains the maximum day count for
	\ the current month.
	1 swap read-dec2-range
	days + 1- >days

	\ Hour, minute and seconds. Count of seconds is allowed to go to
	\ 60 in case of leap seconds (in practice, leap seconds really
	\ occur only at the very end of the day, so this computation is
	\ exact for a real leap second, and a spurious leap second only
	\ implies a one-second shift that we can ignore).
	0 23 read-dec2-range 3600 * >seconds
	0 59 read-dec2-range 60 * seconds + >seconds
	0 60 read-dec2-range seconds + >seconds

	\ At this point, we may have fractional seconds. This should
	\ happen only with GeneralizedTime, but we accept it for UTCTime
	\ too (and, anyway, we ignore these fractional seconds).
	read8 dup `. = if
		drop
		begin read8 dup `0 `9 between? while drop repeat
	then

	\ The time zone should be 'Z', not followed by anything. Other
	\ time zone indications are not DER and thus not supposed to
	\ appear in certificates.
	`Z <> if ERR_X509_BAD_TIME fail then
	close-elt
	days seconds ;

\ Read an INTEGER (tag, length and value). The INTEGER is supposed to be
\ positive; its unsigned big-endian encoding is stored in the provided
\ in-context buffer. Returned value is the decoded length. If the integer
\ did not fit, or the value is negative, then an error is reported.
: read-integer ( lim addr len -- lim dlen )
	rot read-tag 0x02 check-tag-primitive -rot
	read-integer-next ;

\ Identical to read-integer, but the tag has already been read and checked.
: read-integer-next ( lim addr len -- lim dlen )
	dup { addr len origlen }
	read-length-open-elt
	\ Read first byte; sign bit must be 0.
	read8 dup 0x80 >= if ERR_X509_OVERFLOW fail then
	\ Skip leading bytes of value 0. If there are only bytes of
	\ value 0, then return.
	begin dup 0 = while
		drop dup ifnot drop 0 ret then
		read8
	repeat
	\ At that point, we have the first non-zero byte on the stack.
	begin
		len dup ifnot ERR_X509_LIMIT_EXCEEDED fail then 1- >len
		addr set8 addr 1+ >addr
		dup while read8
	repeat
	drop origlen len - ;

\ Read a BOOLEAN value. This should be called immediately after reading
\ the tag.
: read-boolean ( lim constructed value -- lim bool )
	0x01 check-tag-primitive
	read-length 1 <> if ERR_X509_BAD_BOOLEAN fail then
	read8 0<> ;

\ Identify an elliptic curve: read the OID, then check it against the
\ known curve OID.
: read-curve-ID ( lim -- lim curve )
	read-OID ifnot ERR_X509_UNSUPPORTED fail then
	choice
		ansix9p256r1 eqOID uf 23 enduf
		ansix9p384r1 eqOID uf 24 enduf
		ansix9p521r1 eqOID uf 25 enduf
		ERR_X509_UNSUPPORTED fail
	endchoice ;

\ A convenient debug word: print the current data stack contents.
cc: DEBUG ( -- ) {
	extern int printf(const char *fmt, ...);
	uint32_t *p;

	printf("<stack:");
	for (p = &CTX->dp_stack[0]; p != dp; p ++) {
		printf(" %lu", (unsigned long)*p);
	}
	printf(" >\n");
}