ECC_WriteKey

Write an internal EC key to an output string in PEM format.

VBA/VB6 Syntax

nRet = ECC_WriteKey(strOutput, nOutChars, strIntKeyString, nOptions)

C/C++ Syntax

Parameters

szOutput

[out] string of sufficient length to receive the output.

nOutChars

[in] specifying the maximum number of characters to be received.

szIntKeyString

[in] the private or public EC key in an internal key string.

nOptions

[in] choose one of

• PKI_DEFAULT (0) to save the key in the default format (see remarks).
• PKI_KEY_TYPE_PKCS8 to save a NIST/SEC curve private key in PKCS#8 format.

and optionally add any of
PKI_KEY_LEGACY to save a SAFE curve key in "legacy" PKCS#8 v1 format (default is v2 OneAsymmetricKey).
PKI_KEY_COMPRESS to save a NIST/SEC curve public key in compressed format (default is uncompressed) [New in v23.2].

Returns (VBA/C)

If successful, the return value is the number of characters in or required for the output string; otherwise it returns a negative error code.

VBA Wrapper Syntax

.NET Equivalent

Ecc.WriteKey Method

Python Equivalent

C++ (STL) Equivalent

Remarks

This works the same as ECC_SaveKey except it outputs to a string rather than a file. The output is always in textual PEM form. See the remarks for ECC_SaveKey for details of key format.

Use this unencrypted PEM string as input for any ECC, signature, X.509 or ASN.1 function expecting a public or private ECC key. Exercise caution due to the lack of encryption and prefer the internal key string format for private keys to ensure better security.

For the "raw" VBA/C function, the user must allocate an output string buffer szOutput of the required length. Specify a zero nOutChars or an empty string for szOutput to find the required length. ANSI C users must add one to this value when allocating memory.

Example

Dim strCurveName As String
Dim strPriKeyHex As String
Dim strPubKeyHex As String
Dim strPriKeyInt As String
Dim strPubKeyInt As String
Dim strKeyPem As String

' Ref: https://www.ietf.org/archive/id/draft-ietf-cose-cbor-encoded-cert-20.html#appendix-A.1.4
strCurveName = "secp256r1"
strPriKeyHex = "DC66B3415456D649429B53223DF7532B942D6B0E0842C30BCA4C0ACF91547BB2"
strPubKeyHex = "02AE4CDB01F614DEFC7121285FDC7F5C6D1D42C95647F061BA0080DF678867845E"
' Read in keys to ephemeral internal representation
strPriKeyInt = eccReadKeyByCurve(strPriKeyHex, strCurveName)
strPubKeyInt = eccReadKeyByCurve(strPubKeyHex, strCurveName)
' Output private key in PEM form
strKeyPem = eccWriteKey(strPriKeyInt)  ' Default
Debug.Print "PRI KEY default:"
Debug.Print strKeyPem
' Show the ASN.1 structure...
Debug.Print vbCrLf & "ASN.1 structure..."
Debug.Print asn1TextDumpToString(strKeyPem)
' Output public key in compressed form...
strKeyPem = eccWriteKey(strPubKeyInt, PKI_KEY_COMPRESS)
Debug.Print "PUB KEY compressed:"
Debug.Print strKeyPem
' Show the ASN.1 structure...
Debug.Print vbCrLf & "ASN.1 structure..."
Debug.Print asn1TextDumpToString(strKeyPem)

Note that the output values from eccWriteKey are the PEM strings beginning "-----BEGIN EC PRIVATE KEY" and "-----BEGIN PUBLIC KEY". The ASN.1 text dumps show the underlying structure.

TESTING ECC_WRITE_KEY...
PRI KEY default:
-----BEGIN EC PRIVATE KEY-----
MHcCAQEEINxms0FUVtZJQptTIj33UyuULWsOCELDC8pMCs+RVHuyoAoGCCqGSM49
AwEHoUQDQgAErkzbAfYU3vxxIShf3H9cbR1CyVZH8GG6AIDfZ4hnhF7ppp/UiTFJ
2uPTsVQW11MsOHFSuAsN8+GvQIqV0wceWA==
-----END EC PRIVATE KEY-----

ASN.1 structure...
30 77  --SEQUENCE/119 bytes
   02 01  --INTEGER/1 bytes
      01 
   04 20  --OCTETSTRING/32 bytes
      dc 66 b3 41 54 56 d6 49 42 9b 53 22 3d f7 53 2b 
      94 2d 6b 0e 08 42 c3 0b ca 4c 0a cf 91 54 7b b2 
   a0 0a  --[0]/10 bytes
      06 08  --OBJECTIDENTIFIER/8 bytes
         2a 86 48 ce 3d 03 01 07 
         --secp256r1 (1.2.840.10045.3.1.7)
   a1 44  --[1]/68 bytes
      03 42  --BITSTRING/66 bytes
         00 --0 unused bits
         04 ae 4c db 01 f6 14 de fc 71 21 28 5f dc 7f 5c 
         6d 1d 42 c9 56 47 f0 61 ba 00 80 df 67 88 67 84 
         5e e9 a6 9f d4 89 31 49 da e3 d3 b1 54 16 d7 53 
         2c 38 71 52 b8 0b 0d f3 e1 af 40 8a 95 d3 07 1e 
         58 
--(121 bytes)

PUB KEY compressed:
-----BEGIN PUBLIC KEY-----
MDkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDIgACrkzbAfYU3vxxIShf3H9cbR1CyVZH
8GG6AIDfZ4hnhF4=
-----END PUBLIC KEY-----

ASN.1 structure...
30 39  --SEQUENCE/57 bytes
   30 13  --SEQUENCE/19 bytes
      06 07  --OBJECTIDENTIFIER/7 bytes
         2a 86 48 ce 3d 02 01 
         --ecPublicKey (1.2.840.10045.2.1)
      06 08  --OBJECTIDENTIFIER/8 bytes
         2a 86 48 ce 3d 03 01 07 
         --secp256r1 (1.2.840.10045.3.1.7)
   03 22  --BITSTRING/34 bytes
      00 --0 unused bits
      02 ae 4c db 01 f6 14 de fc 71 21 28 5f dc 7f 5c 
      6d 1d 42 c9 56 47 f0 61 ba 00 80 df 67 88 67 84 
      5e 
--(59 bytes)

See Also

ECC_ReadPrivateKey ECC_ReadPublicKey ECC_SaveKey ECC_SaveEncKey ECC_PublicKeyFromPrivate

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