# Converting SM2 Signature Formats (C/C++)

<!--Kit: Crypto Architecture Kit-->
<!--Subsystem: Security-->
<!--Owner: @zxz--3-->
<!--Designer: @lanming-->
<!--Tester: @PAFT-->
<!--Adviser: @zengyawen-->

HarmonyOS supports the conversion between the DER and R|S formats.
You can specify SM2 ciphertext parameters and convert them to DER ciphertext. You can also extract SM2 ciphertext parameters from DER ciphertext.

**Converting Ciphertext Parameters to DER Ciphertext**
1. Call [OH_CryptoEccSignatureSpec_Create](../../reference/apis-crypto-architecture-kit/capi-crypto-signature-h.md#oh_cryptoeccsignaturespec_create) to create an [OH_CryptoEccSignatureSpec](../../reference/apis-crypto-architecture-kit/capi-cryptosignatureapi-oh-cryptoeccsignaturespec.md) object for setting SM2 ciphertext parameters.

2. Call [OH_CryptoEccSignatureSpec_SetRAndS](../../reference/apis-crypto-architecture-kit/capi-crypto-signature-h.md#oh_cryptoeccsignaturespec_setrands) to set the **R** and **S** parameters in the **OH_CryptoEccSignatureSpec** object.

3. Call [OH_CryptoEccSignatureSpec_Encode](../../reference/apis-crypto-architecture-kit/capi-crypto-signature-h.md#oh_cryptoeccsignaturespec_encode) to obtain the converted DER ciphertext.

4. Call [OH_CryptoEccSignatureSpec_Destroy](../../reference/apis-crypto-architecture-kit/capi-crypto-signature-h.md#oh_cryptoeccsignaturespec_destroy) to release the object.

```c++
#include "CryptoArchitectureKit/crypto_common.h"
#include "CryptoArchitectureKit/crypto_asym_key.h"
#include "CryptoArchitectureKit/crypto_signature.h"

static OH_Crypto_ErrCode doTestSm2DataChange()
{
    static unsigned char g_rCoordinate[] = {
        107, 93,  198, 247, 119, 18,  40,  110, 90,  156, 193,
        158, 205, 113, 170, 128, 146, 109, 75,  17,  181, 109,
        110, 91,  149, 5,   110, 233, 209, 78,  229, 96};

    static unsigned char g_sCoordinate[] = {
        45,  153, 88,  82,  104, 221, 226, 43,  174, 21,  122,
        248, 5,   232, 105, 41,  92,  95,  102, 224, 216, 149,
        85,  236, 110, 6,   64,  188, 149, 70,  70,  183};

    // Generate DER signature data using R and S.
    OH_CryptoEccSignatureSpec *spec = NULL;
    Crypto_DataBlob r = {0};
    Crypto_DataBlob s = {0};
    r.data = g_rCoordinate;
    r.len = sizeof(g_rCoordinate);
    s.data = g_sCoordinate;
    s.len = sizeof(g_sCoordinate);
    OH_Crypto_ErrCode ret = OH_CryptoEccSignatureSpec_Create(NULL, &spec);
    if (ret != CRYPTO_SUCCESS) {
        OH_CryptoEccSignatureSpec_Destroy(spec);
        return ret;
    }
    ret = OH_CryptoEccSignatureSpec_SetRAndS(spec, &r, &s);
    if (ret != CRYPTO_SUCCESS) {
        OH_CryptoEccSignatureSpec_Destroy(spec);
        return ret;
    }
    Crypto_DataBlob sig = {0};
    ret = OH_CryptoEccSignatureSpec_Encode(spec, &sig);
    if (ret != CRYPTO_SUCCESS) {
        OH_CryptoEccSignatureSpec_Destroy(spec);
        return ret;
    }
    OH_Crypto_FreeDataBlob(&sig);
    OH_CryptoEccSignatureSpec_Destroy(spec);
    spec = NULL;
    return CRYPTO_SUCCESS;

}
```

**Converting DER Ciphertext to the R and S Parameters**

1. Call [OH_CryptoEccSignatureSpec_Create](../../reference/apis-crypto-architecture-kit/capi-crypto-signature-h.md#oh_cryptoeccsignaturespec_create) to pass the signature data and create a [OH_CryptoEccSignatureSpec](../../reference/apis-crypto-architecture-kit/capi-cryptosignatureapi-oh-cryptoeccsignaturespec.md) object to obtain the converted data.

2. Call [OH_CryptoEccSignatureSpec_GetRAndS](../../reference/apis-crypto-architecture-kit/capi-crypto-signature-h.md#oh_cryptoeccsignaturespec_getrands) to obtain the converted **R** and **S** parameters.

3. Call [OH_CryptoEccSignatureSpec_Destroy](../../reference/apis-crypto-architecture-kit/capi-crypto-signature-h.md#oh_cryptoeccsignaturespec_destroy) to free the memory.

```c++
#include "CryptoArchitectureKit/crypto_common.h"
#include "CryptoArchitectureKit/crypto_asym_key.h"
#include "CryptoArchitectureKit/crypto_signature.h"

static OH_Crypto_ErrCode doSm2GetRS() {
    uint8_t signText[] = {
        0x30, 0x45, 0x02, 0x21, 0x00, 0xab, 0xf8, 0xe2, 0x96, 0x7d, 0x5b, 0x28, 0xfb, 0x9a, 0xbd, 0x05, 0xa6,
        0x81, 0xd6, 0xb1, 0x55, 0x69, 0x22, 0x25, 0xd2, 0xa3, 0x5d, 0xa8, 0xc0, 0x96, 0xe0, 0x1d, 0x38, 0x74,
        0xa0, 0xc9, 0x4f, 0x02, 0x20, 0x20, 0x27, 0x04, 0x7a, 0x31, 0x94, 0xe7, 0x32, 0x61, 0xc3, 0x55, 0xa6,
        0x5e, 0x1e, 0xdd, 0x3d, 0x04, 0x1c, 0x1e, 0x2d, 0x8d, 0x8d, 0x45, 0xca, 0xd9, 0x40, 0xe8, 0x97, 0xcd,
        0x01, 0x18, 0xc5,
    };
    Crypto_DataBlob signBlob = {
        .data = reinterpret_cast<uint8_t *>(signText),
        .len = sizeof(signText)};

    OH_CryptoEccSignatureSpec *eccSignSpec = nullptr;
    OH_Crypto_ErrCode ret = OH_CryptoEccSignatureSpec_Create(&signBlob, &eccSignSpec);
    if (ret != CRYPTO_SUCCESS) {
        return ret;
    }

    Crypto_DataBlob r = {.data = nullptr, .len = 0};
    Crypto_DataBlob s = {.data = nullptr, .len = 0};
    ret = OH_CryptoEccSignatureSpec_GetRAndS(eccSignSpec, &r, &s);
    if (ret != CRYPTO_SUCCESS) {
        OH_CryptoEccSignatureSpec_Destroy(eccSignSpec);
        return ret;
    }
    OH_Crypto_FreeDataBlob(&r);
    OH_Crypto_FreeDataBlob(&s);
    OH_CryptoEccSignatureSpec_Destroy(eccSignSpec);
    return CRYPTO_SUCCESS;
}
```