Remove implicit truncation from i2c_ASN1_BIT_STRING() #1
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This removes the implicit truncation dance from i2c_ASN1_BIT_STRING(), still respecting ASN1_STRING_FLAG_BITS_LEFT and only using the bits from a->flags if that is set. Clean up and reorder the implementation. This changes the encoding of bit strings set via ASN1_BIT_STRING_set() and ASN1_STRING_set() unless that flag is set by the application. The ASN1_BIT_STRING_set() is only used without ASN1_STRING_FLAG_BITS_LEFT by the yubico-piv-tool, where this results in erroneous behavior due to incorrect porting of the code to OpenSSL 1.1. It adds (perhaps somewhat too clever) code to ensure the truncation behavior of ASN1_BIT_STRING_set_bit() is unmodified. It now sets ASN1_STRING_FLAG_BITS_LEFT and so that the resulting DER encoding remains the same, which is needed for things relying on that such as the code setting the keyUsage bits. This is required for correct DER encoding, because: X.680, 22.7: When a "NamedBitList" is used in defining a bitstring type ASN.1 encoding rules are free to add (or remove) arbitrarily any trailing 0 bits to (or from) values that are being encoded or decoded. Application designers should therefore ensure that different semantics are not associated with such vlaues which differ only in the number of trailing 0 bits. X.690, 11.2.2: Where ITU-T Rec. X.680 | ISO/IEC 8824-1, 22.7, applies, the bitstring shall have all trailing 0 bits removed before it is encoded. Note 1 - In the case where a size constraint has been applied, the abstract value delivered by a decoder to the application will be one of those satisfying the size constraint and differing from the transmitted value only in the number of trailing zero bits. Note 2 - If a bitstring value has no 1 bits, then an encoder shall encode the value with a length of 1 and an initial octet set to 0. See openssl/openssl#29711
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This removes the implicit truncation dance from i2c_ASN1_BIT_STRING(), still respecting ASN1_STRING_FLAG_BITS_LEFT and only using the bits from a->flags if that is set. Clean up and reorder the implementation.
This changes the encoding of bit strings set via ASN1_BIT_STRING_set() and ASN1_STRING_set() unless that flag is set by the application. The ASN1_BIT_STRING_set() is only used without ASN1_STRING_FLAG_BITS_LEFT by the yubico-piv-tool, where this results in erroneous behavior due to incorrect porting of the code to OpenSSL 1.1.
It adds (perhaps somewhat too clever) code to ensure the truncation behavior of ASN1_BIT_STRING_set_bit() is unmodified. It now sets ASN1_STRING_FLAG_BITS_LEFT and so that the resulting DER encoding remains the same, which is needed for things relying on that such as the code setting the keyUsage bits. This is required for correct DER encoding, because:
X.680, 22.7:
When a "NamedBitList" is used in defining a bitstring type ASN.1
encoding rules are free to add (or remove) arbitrarily any trailing 0
bits to (or from) values that are being encoded or decoded. Application
designers should therefore ensure that different semantics are not
associated with such vlaues which differ only in the number of trailing
0 bits.
X.690, 11.2.2:
Where ITU-T Rec. X.680 | ISO/IEC 8824-1, 22.7, applies, the bitstring
shall have all trailing 0 bits removed before it is encoded.
Note 1 - In the case where a size constraint has been applied, the
abstract value delivered by a decoder to the application will be one of
those satisfying the size constraint and differing from the transmitted
value only in the number of trailing zero bits.
Note 2 - If a bitstring value has no 1 bits, then an encoder shall
encode the value with a length of 1 and an initial octet set to 0.
See openssl/openssl#29711