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[crypto] Generalise rsa_parse_integer() to asn1_enter_unsigned()

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ECDSA signature values and private keys are fixed-length unsigned
integers modulo N (the group order of the elliptic curve) and are
therefore most naturally represented in ASN.1 using ASN1_OCTET_STRING.

Private key representations do use ASN1_OCTET_STRING, but signature
values tend to use ASN1_INTEGER, which adds no value but does ensure
that the encoding becomes variable-length and requires handling a
pointless extra zero byte if the MSB of the unsigned value happens to
be set.

RSA also makes use of ASN1_INTEGER for modulus and exponent values.
Generalise the existing rsa_parse_integer() to asn1_enter_unsigned()
to allow this code to be reused for ECDSA.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
This commit is contained in:
Michael Brown
2025-12-11 15:02:28 +00:00
parent c7f129fede
commit fb1188936c
3 changed files with 28 additions and 30 deletions
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1
src/include/ipxe/asn1.h
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@@ -488,6 +488,7 @@ extern int asn1_skip_any ( struct asn1_cursor *cursor );
extern int asn1_shrink_any ( struct asn1_cursor *cursor );
extern int asn1_enter_bits ( struct asn1_cursor *cursor,
unsigned int *unused );
extern int asn1_enter_unsigned ( struct asn1_cursor *cursor );
extern int asn1_boolean ( const struct asn1_cursor *cursor );
extern int asn1_integer ( const struct asn1_cursor *cursor, int *value );
extern int asn1_compare ( const struct asn1_cursor *cursor1,