RSA_DecodeMsg

Decodes an EME or EMSA encoded message block according to PKCS#1.

VB6/VBA

Debug.Print "Testing RSA_DecodeMsg ..."
Dim abData() As Byte
Dim abBlock() As Byte
Dim abDigest() As Byte
Dim abDigInfo() As Byte
Dim nDataLen As Long
Dim nBlockLen As Long
Dim nLen As Long
Dim nRet As Long
Dim nOptions As Long

' 0. Create an encoded test block ready for for signing
abData = StrConv("abc", vbFromUnicode)
nDataLen = UBound(abData) - LBound(abData) + 1
nBlockLen = 64
ReDim abBlock(nBlockLen - 1)
nRet = RSA_EncodeMsg(abBlock(0), nBlockLen, abData(0), nDataLen, PKI_EMSIG_PKCSV1_5)
Debug.Print "BLOCK   =" & cnvHexStrFromBytes(abBlock)

' 1. Extract the message digest =SHA1("abc")
nLen = RSA_DecodeMsg(0, 0, abBlock(0), nBlockLen, PKI_EMSIG_PKCSV1_5)
If nLen < 0 Then MsgBox "Decryption Error": Exit Sub
Debug.Print "Message digest is " & nLen & " bytes long"
ReDim abDigest(nLen - 1)
nLen = RSA_DecodeMsg(abDigest(0), nLen, abBlock(0), nBlockLen, PKI_EMSIG_PKCSV1_5)
Debug.Print "HASH    =" & cnvHexStrFromBytes(abDigest)

 ' 2. Extract the full DigestInfo data
nOptions = PKI_EMSIG_PKCSV1_5 + PKI_EMSIG_DIGINFO
nLen = RSA_DecodeMsg(0, 0, abBlock(0), nBlockLen, nOptions)
If nLen < 0 Then MsgBox "Decryption Error": Exit Sub
Debug.Print "DigestInfo is " & nLen & " bytes long"
ReDim abDigest(nLen - 1)
nLen = RSA_DecodeMsg(abDigest(0), nLen, abBlock(0), nBlockLen, nOptions)
Debug.Print "DIGINFO=" & cnvHexStrFromBytes(abDigest)

Output

Testing RSA_DecodeMsg ...
BLOCK   =0001FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF003021300906052B0E03021A05000414A9993E364706816ABA3E25717850C26C9CD0D89D
Message digest is 20 bytes long
HASH    =A9993E364706816ABA3E25717850C26C9CD0D89D
DigestInfo is 35 bytes long
DIGINFO=3021300906052B0E03021A05000414A9993E364706816ABA3E25717850C26C9CD0D89D

VB.NET

Console.WriteLine("Testing RSA_DecodeMsg ...")
Dim abData() As Byte
Dim abBlock() As Byte
Dim abDigest() As Byte
Dim abDigInfo() As Byte
Dim nDataLen As Integer
Dim nBlockLen As Integer

' 0. Create an encoded test block ready for for signing
abData = System.Text.Encoding.Default.GetBytes("abc")
nDataLen = UBound(abData) - LBound(abData) + 1
nBlockLen = 64
abBlock = Rsa.EncodeMsgForSignature(nBlockLen, abData, HashAlgorithm.Sha1)
Console.WriteLine("BLOCK   =" & Cnv.ToHex(abBlock))

' 1. Extract the message digest =SHA1("abc")
abDigest = Rsa.DecodeDigestForSignature(abBlock)
If abDigest.Length = 0 Then
    Console.WriteLine("Decryption Error")
    Exit Sub
End If
Console.WriteLine("Message digest is " & abDigest.Length & " bytes long")
Console.WriteLine("HASH    =" & Cnv.ToHex(abDigest))

' 2. Extract the full DigestInfo data
abDigInfo = Rsa.DecodeDigestForSignature(abBlock, True)
If abDigInfo.Length = 0 Then
    Console.WriteLine("Decryption Error")
    Exit Sub
End If
Console.WriteLine("DigestInfo is " & abDigInfo.Length & " bytes long")
Console.WriteLine("DIGINFO=" & Cnv.ToHex(abDigInfo))

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