RSA Signatures and Endianness

RSA Signatures and Endianness

RSA signatures are represented as large integers, and the byte order (endianness) determines how the bytes of these integers are stored and transmitted. The two common formats are:

1. Big-endian:
   • The most significant byte (MSB) comes first.
   • This is the traditional format used in network protocols and cryptography.
2. Little-endian:
   • The least significant byte (LSB) comes first.
   • Commonly used in systems that follow x86 architecture conventions.

RSA signatures can have their byte ordering defined by the implementation handling them.

Endianness in Cryptographic Libraries and Systems

1. Chilkat:
   • Chilkat RSA is capable of creating and validating both little-endian and big-endian signatures.
   • In addition, when verifying signatures, Chilkat will (internally) auto-recognize the signature byte order and can validate either without having to specify.
2. Microsoft CryptoAPI (CAPI) and CNG:
   • Uses Little-endian format.
   • Microsoft platforms, like Windows, tend to represent multi-byte integers in little-endian order, including RSA signatures and other cryptographic results.
   • Developers working with CryptoAPI or CNG often need to account for this byte ordering when interoperating with big-endian systems.
3. OpenSSL:
   • Uses Big-endian format.
   • OpenSSL adheres to the big-endian convention, which aligns with the majority of cryptographic standards and protocols (e.g., TLS).
4. Java Cryptography Architecture (JCA):
   • Uses Big-endian format.
   • This follows Java’s general handling of cryptographic data, which is typically compatible with OpenSSL and standard protocols.
5. .NET Cryptography:
   • Uses Little-endian format when interacting with Windows CryptoAPI (or underlying system libraries like CNG).
   • However, when converting data for external use (e.g., exporting to DER-encoded formats), it may use big-endian to comply with standards.
6. Bouncy Castle:
   • Uses Big-endian format.
   • Bouncy Castle ensures compatibility with cryptographic standards like PKCS#1 and other common implementations.
7. Cryptographic Standards (e.g., PKCS#1, X.509):
   • Standards like PKCS#1 and X.509 define big-endian ordering for encoded integers in ASN.1 DER format.
   • Any deviation from this (e.g., little-endian) is implementation-specific and needs conversion for interoperability.