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Implement v4.local #11

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Apr 14, 2024
Merged

Implement v4.local #11

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187 changes: 187 additions & 0 deletions testdata/v4.json
View file
Open in desktop
Original file line number Diff line number Diff line change
@@ -0,0 +1,187 @@
{
"name": "PASETO v4 Test Vectors",
"tests": [
{
"name": "4-E-1",
"expect-fail": false,
"key": "707172737475767778797a7b7c7d7e7f808182838485868788898a8b8c8d8e8f",
"nonce": "0000000000000000000000000000000000000000000000000000000000000000",
"token": "v4.local.AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAQAr68PS4AXe7If_ZgesdkUMvSwscFlAl1pk5HC0e8kApeaqMfGo_7OpBnwJOAbY9V7WU6abu74MmcUE8YWAiaArVI8XJ5hOb_4v9RmDkneN0S92dx0OW4pgy7omxgf3S8c3LlQg",
"payload": "{\"data\":\"this is a secret message\",\"exp\":\"2022-01-01T00:00:00+00:00\"}",
"footer": "",
"implicit-assertion": ""
},
{
"name": "4-E-2",
"expect-fail": false,
"key": "707172737475767778797a7b7c7d7e7f808182838485868788898a8b8c8d8e8f",
"nonce": "0000000000000000000000000000000000000000000000000000000000000000",
"token": "v4.local.AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAQAr68PS4AXe7If_ZgesdkUMvS2csCgglvpk5HC0e8kApeaqMfGo_7OpBnwJOAbY9V7WU6abu74MmcUE8YWAiaArVI8XIemu9chy3WVKvRBfg6t8wwYHK0ArLxxfZP73W_vfwt5A",
"payload": "{\"data\":\"this is a hidden message\",\"exp\":\"2022-01-01T00:00:00+00:00\"}",
"footer": "",
"implicit-assertion": ""
},
{
"name": "4-E-3",
"expect-fail": false,
"nonce": "df654812bac492663825520ba2f6e67cf5ca5bdc13d4e7507a98cc4c2fcc3ad8",
"key": "707172737475767778797a7b7c7d7e7f808182838485868788898a8b8c8d8e8f",
"token": "v4.local.32VIErrEkmY4JVILovbmfPXKW9wT1OdQepjMTC_MOtjA4kiqw7_tcaOM5GNEcnTxl60WkwMsYXw6FSNb_UdJPXjpzm0KW9ojM5f4O2mRvE2IcweP-PRdoHjd5-RHCiExR1IK6t6-tyebyWG6Ov7kKvBdkrrAJ837lKP3iDag2hzUPHuMKA",
"payload": "{\"data\":\"this is a secret message\",\"exp\":\"2022-01-01T00:00:00+00:00\"}",
"footer": "",
"implicit-assertion": ""
},
{
"name": "4-E-4",
"expect-fail": false,
"nonce": "df654812bac492663825520ba2f6e67cf5ca5bdc13d4e7507a98cc4c2fcc3ad8",
"key": "707172737475767778797a7b7c7d7e7f808182838485868788898a8b8c8d8e8f",
"token": "v4.local.32VIErrEkmY4JVILovbmfPXKW9wT1OdQepjMTC_MOtjA4kiqw7_tcaOM5GNEcnTxl60WiA8rd3wgFSNb_UdJPXjpzm0KW9ojM5f4O2mRvE2IcweP-PRdoHjd5-RHCiExR1IK6t4gt6TiLm55vIH8c_lGxxZpE3AWlH4WTR0v45nsWoU3gQ",
"payload": "{\"data\":\"this is a hidden message\",\"exp\":\"2022-01-01T00:00:00+00:00\"}",
"footer": "",
"implicit-assertion": ""
},
{
"name": "4-E-5",
"expect-fail": false,
"nonce": "df654812bac492663825520ba2f6e67cf5ca5bdc13d4e7507a98cc4c2fcc3ad8",
"key": "707172737475767778797a7b7c7d7e7f808182838485868788898a8b8c8d8e8f",
"token": "v4.local.32VIErrEkmY4JVILovbmfPXKW9wT1OdQepjMTC_MOtjA4kiqw7_tcaOM5GNEcnTxl60WkwMsYXw6FSNb_UdJPXjpzm0KW9ojM5f4O2mRvE2IcweP-PRdoHjd5-RHCiExR1IK6t4x-RMNXtQNbz7FvFZ_G-lFpk5RG3EOrwDL6CgDqcerSQ.eyJraWQiOiJ6VmhNaVBCUDlmUmYyc25FY1Q3Z0ZUaW9lQTlDT2NOeTlEZmdMMVc2MGhhTiJ9",
"payload": "{\"data\":\"this is a secret message\",\"exp\":\"2022-01-01T00:00:00+00:00\"}",
"footer": "{\"kid\":\"zVhMiPBP9fRf2snEcT7gFTioeA9COcNy9DfgL1W60haN\"}",
"implicit-assertion": ""
},
{
"name": "4-E-6",
"expect-fail": false,
"nonce": "df654812bac492663825520ba2f6e67cf5ca5bdc13d4e7507a98cc4c2fcc3ad8",
"key": "707172737475767778797a7b7c7d7e7f808182838485868788898a8b8c8d8e8f",
"token": "v4.local.32VIErrEkmY4JVILovbmfPXKW9wT1OdQepjMTC_MOtjA4kiqw7_tcaOM5GNEcnTxl60WiA8rd3wgFSNb_UdJPXjpzm0KW9ojM5f4O2mRvE2IcweP-PRdoHjd5-RHCiExR1IK6t6pWSA5HX2wjb3P-xLQg5K5feUCX4P2fpVK3ZLWFbMSxQ.eyJraWQiOiJ6VmhNaVBCUDlmUmYyc25FY1Q3Z0ZUaW9lQTlDT2NOeTlEZmdMMVc2MGhhTiJ9",
"payload": "{\"data\":\"this is a hidden message\",\"exp\":\"2022-01-01T00:00:00+00:00\"}",
"footer": "{\"kid\":\"zVhMiPBP9fRf2snEcT7gFTioeA9COcNy9DfgL1W60haN\"}",
"implicit-assertion": ""
},
{
"name": "4-E-7",
"expect-fail": false,
"nonce": "df654812bac492663825520ba2f6e67cf5ca5bdc13d4e7507a98cc4c2fcc3ad8",
"key": "707172737475767778797a7b7c7d7e7f808182838485868788898a8b8c8d8e8f",
"token": "v4.local.32VIErrEkmY4JVILovbmfPXKW9wT1OdQepjMTC_MOtjA4kiqw7_tcaOM5GNEcnTxl60WkwMsYXw6FSNb_UdJPXjpzm0KW9ojM5f4O2mRvE2IcweP-PRdoHjd5-RHCiExR1IK6t40KCCWLA7GYL9KFHzKlwY9_RnIfRrMQpueydLEAZGGcA.eyJraWQiOiJ6VmhNaVBCUDlmUmYyc25FY1Q3Z0ZUaW9lQTlDT2NOeTlEZmdMMVc2MGhhTiJ9",
"payload": "{\"data\":\"this is a secret message\",\"exp\":\"2022-01-01T00:00:00+00:00\"}",
"footer": "{\"kid\":\"zVhMiPBP9fRf2snEcT7gFTioeA9COcNy9DfgL1W60haN\"}",
"implicit-assertion": "{\"test-vector\":\"4-E-7\"}"
},
{
"name": "4-E-8",
"expect-fail": false,
"nonce": "df654812bac492663825520ba2f6e67cf5ca5bdc13d4e7507a98cc4c2fcc3ad8",
"key": "707172737475767778797a7b7c7d7e7f808182838485868788898a8b8c8d8e8f",
"token": "v4.local.32VIErrEkmY4JVILovbmfPXKW9wT1OdQepjMTC_MOtjA4kiqw7_tcaOM5GNEcnTxl60WiA8rd3wgFSNb_UdJPXjpzm0KW9ojM5f4O2mRvE2IcweP-PRdoHjd5-RHCiExR1IK6t5uvqQbMGlLLNYBc7A6_x7oqnpUK5WLvj24eE4DVPDZjw.eyJraWQiOiJ6VmhNaVBCUDlmUmYyc25FY1Q3Z0ZUaW9lQTlDT2NOeTlEZmdMMVc2MGhhTiJ9",
"payload": "{\"data\":\"this is a hidden message\",\"exp\":\"2022-01-01T00:00:00+00:00\"}",
"footer": "{\"kid\":\"zVhMiPBP9fRf2snEcT7gFTioeA9COcNy9DfgL1W60haN\"}",
"implicit-assertion": "{\"test-vector\":\"4-E-8\"}"
},
{
"name": "4-E-9",
"expect-fail": false,
"nonce": "df654812bac492663825520ba2f6e67cf5ca5bdc13d4e7507a98cc4c2fcc3ad8",
"key": "707172737475767778797a7b7c7d7e7f808182838485868788898a8b8c8d8e8f",
"token": "v4.local.32VIErrEkmY4JVILovbmfPXKW9wT1OdQepjMTC_MOtjA4kiqw7_tcaOM5GNEcnTxl60WiA8rd3wgFSNb_UdJPXjpzm0KW9ojM5f4O2mRvE2IcweP-PRdoHjd5-RHCiExR1IK6t6tybdlmnMwcDMw0YxA_gFSE_IUWl78aMtOepFYSWYfQA.YXJiaXRyYXJ5LXN0cmluZy10aGF0LWlzbid0LWpzb24",
"payload": "{\"data\":\"this is a hidden message\",\"exp\":\"2022-01-01T00:00:00+00:00\"}",
"footer": "arbitrary-string-that-isn't-json",
"implicit-assertion": "{\"test-vector\":\"4-E-9\"}"
},
{
"name": "4-S-1",
"expect-fail": false,
"public-key": "1eb9dbbbbc047c03fd70604e0071f0987e16b28b757225c11f00415d0e20b1a2",
"secret-key": "b4cbfb43df4ce210727d953e4a713307fa19bb7d9f85041438d9e11b942a37741eb9dbbbbc047c03fd70604e0071f0987e16b28b757225c11f00415d0e20b1a2",
"secret-key-seed": "b4cbfb43df4ce210727d953e4a713307fa19bb7d9f85041438d9e11b942a3774",
"secret-key-pem": "-----BEGIN PRIVATE KEY-----\nMC4CAQAwBQYDK2VwBCIEILTL+0PfTOIQcn2VPkpxMwf6Gbt9n4UEFDjZ4RuUKjd0\n-----END PRIVATE KEY-----",
"public-key-pem": "-----BEGIN PUBLIC KEY-----\nMCowBQYDK2VwAyEAHrnbu7wEfAP9cGBOAHHwmH4Wsot1ciXBHwBBXQ4gsaI=\n-----END PUBLIC KEY-----",
"token": "v4.public.eyJkYXRhIjoidGhpcyBpcyBhIHNpZ25lZCBtZXNzYWdlIiwiZXhwIjoiMjAyMi0wMS0wMVQwMDowMDowMCswMDowMCJ9bg_XBBzds8lTZShVlwwKSgeKpLT3yukTw6JUz3W4h_ExsQV-P0V54zemZDcAxFaSeef1QlXEFtkqxT1ciiQEDA",
"payload": "{\"data\":\"this is a signed message\",\"exp\":\"2022-01-01T00:00:00+00:00\"}",
"footer": "",
"implicit-assertion": ""
},
{
"name": "4-S-2",
"expect-fail": false,
"public-key": "1eb9dbbbbc047c03fd70604e0071f0987e16b28b757225c11f00415d0e20b1a2",
"secret-key": "b4cbfb43df4ce210727d953e4a713307fa19bb7d9f85041438d9e11b942a37741eb9dbbbbc047c03fd70604e0071f0987e16b28b757225c11f00415d0e20b1a2",
"secret-key-seed": "b4cbfb43df4ce210727d953e4a713307fa19bb7d9f85041438d9e11b942a3774",
"secret-key-pem": "-----BEGIN PRIVATE KEY-----\nMC4CAQAwBQYDK2VwBCIEILTL+0PfTOIQcn2VPkpxMwf6Gbt9n4UEFDjZ4RuUKjd0\n-----END PRIVATE KEY-----",
"public-key-pem": "-----BEGIN PUBLIC KEY-----\nMCowBQYDK2VwAyEAHrnbu7wEfAP9cGBOAHHwmH4Wsot1ciXBHwBBXQ4gsaI=\n-----END PUBLIC KEY-----",
"token": "v4.public.eyJkYXRhIjoidGhpcyBpcyBhIHNpZ25lZCBtZXNzYWdlIiwiZXhwIjoiMjAyMi0wMS0wMVQwMDowMDowMCswMDowMCJ9v3Jt8mx_TdM2ceTGoqwrh4yDFn0XsHvvV_D0DtwQxVrJEBMl0F2caAdgnpKlt4p7xBnx1HcO-SPo8FPp214HDw.eyJraWQiOiJ6VmhNaVBCUDlmUmYyc25FY1Q3Z0ZUaW9lQTlDT2NOeTlEZmdMMVc2MGhhTiJ9",
"payload": "{\"data\":\"this is a signed message\",\"exp\":\"2022-01-01T00:00:00+00:00\"}",
"footer": "{\"kid\":\"zVhMiPBP9fRf2snEcT7gFTioeA9COcNy9DfgL1W60haN\"}",
"implicit-assertion": ""
},
{
"name": "4-S-3",
"expect-fail": false,
"public-key": "1eb9dbbbbc047c03fd70604e0071f0987e16b28b757225c11f00415d0e20b1a2",
"secret-key": "b4cbfb43df4ce210727d953e4a713307fa19bb7d9f85041438d9e11b942a37741eb9dbbbbc047c03fd70604e0071f0987e16b28b757225c11f00415d0e20b1a2",
"secret-key-seed": "b4cbfb43df4ce210727d953e4a713307fa19bb7d9f85041438d9e11b942a3774",
"secret-key-pem": "-----BEGIN PRIVATE KEY-----\nMC4CAQAwBQYDK2VwBCIEILTL+0PfTOIQcn2VPkpxMwf6Gbt9n4UEFDjZ4RuUKjd0\n-----END PRIVATE KEY-----",
"public-key-pem": "-----BEGIN PUBLIC KEY-----\nMCowBQYDK2VwAyEAHrnbu7wEfAP9cGBOAHHwmH4Wsot1ciXBHwBBXQ4gsaI=\n-----END PUBLIC KEY-----",
"token": "v4.public.eyJkYXRhIjoidGhpcyBpcyBhIHNpZ25lZCBtZXNzYWdlIiwiZXhwIjoiMjAyMi0wMS0wMVQwMDowMDowMCswMDowMCJ9NPWciuD3d0o5eXJXG5pJy-DiVEoyPYWs1YSTwWHNJq6DZD3je5gf-0M4JR9ipdUSJbIovzmBECeaWmaqcaP0DQ.eyJraWQiOiJ6VmhNaVBCUDlmUmYyc25FY1Q3Z0ZUaW9lQTlDT2NOeTlEZmdMMVc2MGhhTiJ9",
"payload": "{\"data\":\"this is a signed message\",\"exp\":\"2022-01-01T00:00:00+00:00\"}",
"footer": "{\"kid\":\"zVhMiPBP9fRf2snEcT7gFTioeA9COcNy9DfgL1W60haN\"}",
"implicit-assertion": "{\"test-vector\":\"4-S-3\"}"
},
{
"name": "4-F-1",
"expect-fail": true,
"public-key": "1eb9dbbbbc047c03fd70604e0071f0987e16b28b757225c11f00415d0e20b1a2",
"secret-key": "b4cbfb43df4ce210727d953e4a713307fa19bb7d9f85041438d9e11b942a37741eb9dbbbbc047c03fd70604e0071f0987e16b28b757225c11f00415d0e20b1a2",
"secret-key-seed": "b4cbfb43df4ce210727d953e4a713307fa19bb7d9f85041438d9e11b942a3774",
"secret-key-pem": "-----BEGIN PRIVATE KEY-----\nMC4CAQAwBQYDK2VwBCIEILTL+0PfTOIQcn2VPkpxMwf6Gbt9n4UEFDjZ4RuUKjd0\n-----END PRIVATE KEY-----",
"public-key-pem": "-----BEGIN PUBLIC KEY-----\nMCowBQYDK2VwAyEAHrnbu7wEfAP9cGBOAHHwmH4Wsot1ciXBHwBBXQ4gsaI=\n-----END PUBLIC KEY-----",
"token": "v4.local.vngXfCISbnKgiP6VWGuOSlYrFYU300fy9ijW33rznDYgxHNPwWluAY2Bgb0z54CUs6aYYkIJ-bOOOmJHPuX_34Agt_IPlNdGDpRdGNnBz2MpWJvB3cttheEc1uyCEYltj7wBQQYX.YXJiaXRyYXJ5LXN0cmluZy10aGF0LWlzbid0LWpzb24",
"payload": null,
"footer": "arbitrary-string-that-isn't-json",
"implicit-assertion": "{\"test-vector\":\"4-F-1\"}"
},
{
"name": "4-F-2",
"expect-fail": true,
"nonce": "df654812bac492663825520ba2f6e67cf5ca5bdc13d4e7507a98cc4c2fcc3ad8",
"key": "707172737475767778797a7b7c7d7e7f808182838485868788898a8b8c8d8e8f",
"token": "v4.public.eyJpbnZhbGlkIjoidGhpcyBzaG91bGQgbmV2ZXIgZGVjb2RlIn22Sp4gjCaUw0c7EH84ZSm_jN_Qr41MrgLNu5LIBCzUr1pn3Z-Wukg9h3ceplWigpoHaTLcwxj0NsI1vjTh67YB.eyJraWQiOiJ6VmhNaVBCUDlmUmYyc25FY1Q3Z0ZUaW9lQTlDT2NOeTlEZmdMMVc2MGhhTiJ9",
"payload": null,
"footer": "{\"kid\":\"zVhMiPBP9fRf2snEcT7gFTioeA9COcNy9DfgL1W60haN\"}",
"implicit-assertion": "{\"test-vector\":\"4-F-2\"}"
},
{
"name": "4-F-3",
"expect-fail": true,
"nonce": "26f7553354482a1d91d4784627854b8da6b8042a7966523c2b404e8dbbe7f7f2",
"key": "707172737475767778797a7b7c7d7e7f808182838485868788898a8b8c8d8e8f",
"token": "v3.local.23e_2PiqpQBPvRFKzB0zHhjmxK3sKo2grFZRRLM-U7L0a8uHxuF9RlVz3Ic6WmdUUWTxCaYycwWV1yM8gKbZB2JhygDMKvHQ7eBf8GtF0r3K0Q_gF1PXOxcOgztak1eD1dPe9rLVMSgR0nHJXeIGYVuVrVoLWQ.YXJiaXRyYXJ5LXN0cmluZy10aGF0LWlzbid0LWpzb24",
"payload": null,
"footer": "arbitrary-string-that-isn't-json",
"implicit-assertion": "{\"test-vector\":\"4-F-3\"}"
},
{
"name": "4-F-4",
"expect-fail": true,
"nonce": "df654812bac492663825520ba2f6e67cf5ca5bdc13d4e7507a98cc4c2fcc3ad8",
"key": "707172737475767778797a7b7c7d7e7f808182838485868788898a8b8c8d8e8f",
"token": "v4.local.AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAQAr68PS4AXe7If_ZgesdkUMvSwscFlAl1pk5HC0e8kApeaqMfGo_7OpBnwJOAbY9V7WU6abu74MmcUE8YWAiaArVI8XJ5hOb_4v9RmDkneN0S92dx0OW4pgy7omxgf3S8c3LlQh",
"payload": null,
"footer": "",
"implicit-assertion": ""
},
{
"name": "4-F-5",
"expect-fail": true,
"nonce": "df654812bac492663825520ba2f6e67cf5ca5bdc13d4e7507a98cc4c2fcc3ad8",
"key": "707172737475767778797a7b7c7d7e7f808182838485868788898a8b8c8d8e8f",
"token": "v4.local.32VIErrEkmY4JVILovbmfPXKW9wT1OdQepjMTC_MOtjA4kiqw7_tcaOM5GNEcnTxl60WkwMsYXw6FSNb_UdJPXjpzm0KW9ojM5f4O2mRvE2IcweP-PRdoHjd5-RHCiExR1IK6t4x-RMNXtQNbz7FvFZ_G-lFpk5RG3EOrwDL6CgDqcerSQ==.eyJraWQiOiJ6VmhNaVBCUDlmUmYyc25FY1Q3Z0ZUaW9lQTlDT2NOeTlEZmdMMVc2MGhhTiJ9",
"payload": null,
"footer": "{\"kid\":\"zVhMiPBP9fRf2snEcT7gFTioeA9COcNy9DfgL1W60haN\"}",
"implicit-assertion": ""
}
]
}
193 changes: 193 additions & 0 deletions v4loc.go
View file
Open in desktop
Original file line number Diff line number Diff line change
@@ -0,0 +1,193 @@
package paseto

import (
"crypto/hmac"
"crypto/rand"
"errors"
"fmt"
"io"
"strings"

"golang.org/x/crypto/blake2b"
"golang.org/x/crypto/chacha20"
)

const (
v4locHeader = "v4.local."
v4locKey = 32
v4locNonce = 32
v4locMac = 32
v4locKDF = 32
)

func V4Encrypt(key []byte, payload, footer any, implicit string, randBytes []byte) (string, error) {
payloadBytes, err := toBytes(payload)
if err != nil {
return "", fmt.Errorf("encode payload: %w", err)
}

footerBytes, err := toBytes(footer)
if err != nil {
return "", fmt.Errorf("encode footer: %w", err)
}

if randBytes == nil {
randBytes = make([]byte, v4locNonce)
if _, err := io.ReadFull(rand.Reader, randBytes); err != nil {
return "", fmt.Errorf("read from crypto/rand.Reader: %w", err)
}
}

// step 0.
m := payloadBytes
k := key
f := footerBytes
i := []byte(implicit)

// step 1.
if !constTimeEq(int32(len(k)), v4locKey) {
return "", errors.New("bad key")
}

// step 2.
h := []byte(v4locHeader)

// step 3.
n := randBytes
if n == nil {
n = make([]byte, v4locNonce)
if _, err := io.ReadFull(rand.Reader, n); err != nil {
return "", fmt.Errorf("read from crypto/rand.Reader: %w", err)
}
}

// step 4.
ek, n2, ak, err := v4locSplitKey(key, n)
if err != nil {
return "", fmt.Errorf("create enc and auth keys: %w", err)
}

// step 5.
c := make([]byte, len(m))

ciph, err := chacha20.NewUnauthenticatedCipher(ek, n2)
if err != nil {
return "", fmt.Errorf("create chacha20 cipher: %w", err)
}
ciph.XORKeyStream(c, m)

// step 6.
preAuth := pae(h, n, c, f, i)

// step 7.
mac, err := blake2b.New(v4locMac, ak)
if err != nil {
return "", fmt.Errorf("unable to in initialize MAC kdf: %w", err)
}
mac.Write(preAuth)
t := mac.Sum(nil)

// step 8.
body := make([]byte, 0, len(n)+len(c)+len(t))
body = append(body, n...)
body = append(body, c...)
body = append(body, t...)

return buildToken(h, body, f), nil
}

func V4Decrypt(token string, key []byte, payload, footer any, implicit string) error {
// step 0.
k := key
i := []byte(implicit)

// step 1.
// step 2.

// step 3.
if !strings.HasPrefix(token, v4locHeader) {
return ErrIncorrectTokenFormat
}
h := []byte(v4locHeader)

// step 4.
body, footerBytes, err := splitToken(token, v4locHeader)
if err != nil {
return fmt.Errorf("decode token: %w", err)
}
if len(body) < v4locNonce+v4locMac {
return ErrIncorrectTokenFormat
}
f := footerBytes
n := body[:v4locNonce]
c := body[v4locNonce : len(body)-v4locMac]
t := body[v4locNonce+len(c):]

// step 5.
ek, n2, ak, err := v4locSplitKey(k, n)
if err != nil {
return fmt.Errorf("create enc and auth keys: %w", err)
}

// step 6.
preAuth := pae(h, n, c, f, i)

// step 7.
hasher, err := blake2b.New(v4locMac, ak)
if err != nil {
return fmt.Errorf("create blake2b hash: %w", err)
}
hasher.Write(preAuth)
t2 := hasher.Sum(nil)

// step 8.
if !hmac.Equal(t, t2) {
return ErrInvalidTokenAuth
}

// step 9.
ciph, err := chacha20.NewUnauthenticatedCipher(ek, n2)
if err != nil {
return fmt.Errorf("create chacha20 cipher: %w", err)
}

p := make([]byte, len(c))
ciph.XORKeyStream(p, c)

// step 10.
if payload != nil {
if err := fromBytes(p, payload); err != nil {
return fmt.Errorf("decode payload: %w", err)
}
}

if footer != nil {
if err := fromBytes(f, footer); err != nil {
return fmt.Errorf("decode footer: %w", err)
}
}
return nil
}

func v4locSplitKey(key, n []byte) (ek, n2, ak []byte, err error) {
encKDF, err := blake2b.New(56, key)
if err != nil {
return nil, nil, nil, err
}

encKDF.Write([]byte("paseto-encryption-key"))
encKDF.Write(n)
tmp := encKDF.Sum(nil)
ek, n2 = tmp[:v4locKDF], tmp[v4locKDF:]

authKDF, err := blake2b.New(32, key)
if err != nil {
return nil, nil, nil, err
}

authKDF.Write([]byte("paseto-auth-key-for-aead"))
authKDF.Write(n)
ak = authKDF.Sum(nil)

return ek, n2, ak, nil
}
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