FormatPkcs1Sign Method

FormatPkcs1Sign(ReadOnlySpan<Byte>, Int32, Int32)

Build the digest into a PKCS #1 v1.5 formatted block for signing (see RFC 8017).

C#
public static byte[] FormatPkcs1Sign(ReadOnlySpan<byte> digest, int digestAlgorithm, int keySizeBits)

Parameters

Type Name Description
System.ReadOnlySpan<System.Byte> digest

The message digest value to format.
System.Int32 digestAlgorithm

The algorithm used to compute the message digest. It must be one of the digest algorithms defined in this class: RsaFormat.Sha1, RsaFormat.Sha256, and so on.
System.Int32 keySizeBits

The size of the key used, in bits. This value must be one of the RsaFormat.KeySizeBits-x- values.

Returns

System.Byte[]

A new byte array containing the formatted digest.

Exceptions

Type Condition
System.ArgumentException

The digest length does not match the digestAlgorithm, or the digestAlgorithm is not supported, or the keySizeBits is not supported.

Remarks

This method will build a new buffer that is keySizeBits long and contains the following data.

  00 || 01 || FF FF ... FF || 00 || DigestInfo(digest)

The DigestInfo is the DER encoding of the ASN.1 definition

   DigestInfo ::= SEQUENCE {
       digestAlgorithm   DigestAlgorithm,
       digest            OCTET STRING
   }

This method supports only the following digest algorithms. Note that the length of the digest given must match the digestAlgorithm, otherwise the method will throw an exception. 

  SHA-1     RsaFormat.Sha1     20 bytes
  SHA-256   RsaFormat.Sha256   32 bytes
  SHA-384   RsaFormat.Sha384   48 bytes
  SHA-512   RsaFormat.Sha512   64 bytes

This method supports only keySizeBits values that are defined in this class as KeySizeBits-x-, such as RsaFormat.KeySizeBits1024 (x=1024). You can use one of these values or simply the actual key size in bits. For example, if the key size in bits is 1024, then either RsaFormat.KeySizeBits1024 or 1024 are valid input to this method.

For example, if the digest is 32 bytes long, the digestAlgorithm is RsaFormat.Sha256 and the keySizeBits is 1024, the result of this method will look like the following. 

   00 01 FF FF FF FF FF FF FF FF FF FF FF FF FF FF
   FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF
   FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF
   FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF
   FF FF FF FF FF FF FF FF FF FF FF FF 00 30 31 30
   0d 06 09 60 86 48 01 65 03 04 02 01 05 00 04 20
   01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F 10
   11 12 13 14 15 16 17 18 19 1A 1B 1C 1D 1E 1F 20