Functions/New-JwtSignature.ps1
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function New-JwtSignature { <# .SYNOPSIS Generates an RSA or HMAC signature for the header and payload of a JSON Web Token. .DESCRIPTION Generates an RSA or HMAC signature for the incoming value in the JsonWebToken parameter. The value passed to the JsonWebToken parameter should contain only the header and payload of a JSON Web Token with the appropriate algorithm being defined in the "alg" claim of the header. .PARAMETER JsonWebToken Contains the JWT minus the signature (encoded header and payload seperated by a period) to generate the HMAC signature for. .PARAMETER HashAlgorithm The hash algorithim for the signature. Acceptable values are SHA256, SHA384, and SHA512. .PARAMETER SigningCertificate The certificate containing the private key that will sign the JSON Web Token. .PARAMETER Key The secret key used to generate the HMAC signature. .PARAMETER SkipJwtStructureTest Skips testing the incoming JWT for structural validity. .EXAMPLE $jwtSansSig = "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOjEyMzQ1Njc4OTAsIm5hbWUiOiJKb2huIERvZSIsImFkbWluIjp0cnVlfQ" $myCert = Get-Item -Path Cert:\CurrentUser\my\690F0B1D51BD88AEEA1E374B22BBA7BDAB1BE84B $rsaSig = New-JwtSignature -JsonWebToken $jwtSansSig -HashAlgorithm SHA256 -SigningCertificate $myCert $jws = "{0}.{1}" -f $jwtSansSig, $rsaSig Produces an SHA256 RSA signature for the JWT in the $jwtSansSig variable and constructs a JSON Web Signature (JWS) defined in the $jws variable. .EXAMPLE $jwtSansSig = "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiYWRtaW4iOnRydWV9" $secret = "super secret key!" $hmacSig = New-JwtSignature -JsonWebToken $jwtSansSig -Key $secret -HashAlgorithm SHA256 $jws = "{0}.{1}" -f $jwtSansSig, $hmacSig Validates the two part passed JWT, produces an HMACSHA256 signature for the JWT in the $jwtSansSig variable, and constructs a JSON Web Signature (JWS) defined in the $jws variable. .NOTES The resulting HMAC SHA-256, SHA-384, or SHA-512 algorithm is base64url encoded to be compliant with JWT standard RFC 7519. .OUTPUTS System.String An digital signature is returned as a base64 URL encoded string. .LINK https://tools.ietf.org/html/rfc7519 https://en.wikipedia.org/wiki/RSA_(cryptosystem) https://en.wikipedia.org/wiki/HMAC #> [CmdletBinding()] [OutputType([System.String])] Param ( [Parameter(Mandatory=$true,ValueFromPipeline=$true,ValueFromPipelineByPropertyName=$true,Position=0)] [ValidateLength(16,131072)][Alias("JWT", "Token")][String]$JsonWebToken, [Parameter(Mandatory=$false,Position=1)] [ValidateSet("SHA256","SHA384","SHA512")] [String]$HashAlgorithm="SHA256", [Parameter(Mandatory=$true,ParameterSetName="RSA",Position=2)][Alias("Certificate", "Cert")] [System.Security.Cryptography.X509Certificates.X509Certificate2]$SigningCertificate, [Parameter(Mandatory = $true, ParameterSetName = "HMAC", Position = 3)] [ValidateLength(1, 32768)] [String]$Key, [Parameter(Mandatory=$false,ValueFromPipeline=$false,Position=4)][Switch]$SkipJwtStructureTest ) BEGIN { $decodeExceptionMessage = "Unable to decode JWT." $ArgumentException = New-Object -TypeName ArgumentException -ArgumentList $decodeExceptionMessage } PROCESS { [string]$jwtSignature = "" if (-not($PSBoundParameters.ContainsKey("SkipJwtStructureTest"))) { [bool]$isValidJwt = Test-JwtStructure -JsonWebToken $JsonWebToken if (-not($isValidJwt)) { Write-Error -Exception $ArgumentException -Category InvalidArgument -ErrorAction Stop } } if ($PSCmdlet.ParameterSetName -eq "HMAC") { [byte[]]$secretBytes = [Encoding]::ASCII.GetBytes($Key) [HMAC]$hmacSha = $null switch ($HashAlgorithm) { "SHA256" { $hmacSha = [HMACSHA256]::new($secretBytes) } "SHA384" { $hmacSha = [HMACSHA384]::new($secretBytes) } "SHA512" { $hmacSha = [HMACSHA512]::new($secretBytes) } default { $hmacSha = [HMACSHA256]::new($secretBytes) } } $hashBytes = $hmacsha.ComputeHash([Text.Encoding]::ASCII.GetBytes($JsonWebToken)) $jwtSignature = ConvertTo-Base64UrlEncodedString -Bytes $hashBytes $hmacSha.Dispose() } else { try { $jwtSignature = New-JwtRsaSignature -JsonWebToken $JsonWebToken -SigningCertificate $SigningCertificate -HashAlgorithm $HashAlgorithm } catch { $cryptographicExceptionMessage = $_.Exception.Message $CryptographicException = New-Object -TypeName System.Security.Cryptography.CryptographicException -ArgumentList $cryptographicExceptionMessage Write-Error -Exception $CryptographicException -Category SecurityError -ErrorAction Stop } } return $jwtSignature } } |