Experiment with runtime/secret

Makefile

@@ -1,6 +1,8 @@
 VERSION = 0.2.0
 LDFLAGS = -ldflags='-s -w' -trimpath
 
+export GOEXPERIMENT=runtimesecret
+
 linux_amd64: export GOOS=linux
 linux_amd64: export GOARCH=amd64
 linux_arm: export GOOS=linux
@@ -19,10 +21,13 @@ windows_arm: export GOARCH=arm
 windows_arm64: export GOOS=windows
 windows_arm64: export GOARCH=arm64
 
-.PHONY: all linux_amd64 linux_arm linux_arm64 darwin_amd64 darwin_arm64 windows_amd64 windows_arm windows_arm64 clean
+.PHONY: all build linux_amd64 linux_arm linux_arm64 darwin_amd64 darwin_arm64 windows_amd64 windows_arm windows_arm64 clean
 
 all: linux_amd64 linux_arm linux_arm64 darwin_amd64 darwin_arm64 windows_amd64 windows_arm windows_arm64
 
+build:
+	go build
+
 linux_amd64:
 	go build $(LDFLAGS)
 	mkdir -p release

go.mod

@@ -1,32 +1,31 @@
 module github.com/stefansundin/shrimp
 
-go 1.21
-
-toolchain go1.23.0
+go 1.24.0
 
 require (
-	github.com/aws/aws-sdk-go-v2 v1.30.4
-	github.com/aws/aws-sdk-go-v2/config v1.27.31
-	github.com/aws/aws-sdk-go-v2/credentials v1.17.30
-	github.com/aws/aws-sdk-go-v2/service/s3 v1.61.0
-	github.com/aws/smithy-go v1.20.4
+	github.com/aws/aws-sdk-go-v2 v1.41.1
+	github.com/aws/aws-sdk-go-v2/config v1.32.7
+	github.com/aws/aws-sdk-go-v2/credentials v1.19.7
+	github.com/aws/aws-sdk-go-v2/service/s3 v1.96.0
+	github.com/aws/smithy-go v1.24.0
 	github.com/pkg/term v1.1.0
 	github.com/stefansundin/go-zflag v1.1.1
-	golang.org/x/sys v0.24.0
+	golang.org/x/sys v0.41.0
 )
 
 require (
-	github.com/aws/aws-sdk-go-v2/aws/protocol/eventstream v1.6.4 // indirect
-	github.com/aws/aws-sdk-go-v2/feature/ec2/imds v1.16.12 // indirect
-	github.com/aws/aws-sdk-go-v2/internal/configsources v1.3.16 // indirect
-	github.com/aws/aws-sdk-go-v2/internal/endpoints/v2 v2.6.16 // indirect
-	github.com/aws/aws-sdk-go-v2/internal/ini v1.8.1 // indirect
-	github.com/aws/aws-sdk-go-v2/internal/v4a v1.3.16 // indirect
-	github.com/aws/aws-sdk-go-v2/service/internal/accept-encoding v1.11.4 // indirect
-	github.com/aws/aws-sdk-go-v2/service/internal/checksum v1.3.18 // indirect
-	github.com/aws/aws-sdk-go-v2/service/internal/presigned-url v1.11.18 // indirect
-	github.com/aws/aws-sdk-go-v2/service/internal/s3shared v1.17.16 // indirect
-	github.com/aws/aws-sdk-go-v2/service/sso v1.22.5 // indirect
-	github.com/aws/aws-sdk-go-v2/service/ssooidc v1.26.5 // indirect
-	github.com/aws/aws-sdk-go-v2/service/sts v1.30.5 // indirect
+	github.com/aws/aws-sdk-go-v2/aws/protocol/eventstream v1.7.4 // indirect
+	github.com/aws/aws-sdk-go-v2/feature/ec2/imds v1.18.17 // indirect
+	github.com/aws/aws-sdk-go-v2/internal/configsources v1.4.17 // indirect
+	github.com/aws/aws-sdk-go-v2/internal/endpoints/v2 v2.7.17 // indirect
+	github.com/aws/aws-sdk-go-v2/internal/ini v1.8.4 // indirect
+	github.com/aws/aws-sdk-go-v2/internal/v4a v1.4.17 // indirect
+	github.com/aws/aws-sdk-go-v2/service/internal/accept-encoding v1.13.4 // indirect
+	github.com/aws/aws-sdk-go-v2/service/internal/checksum v1.9.8 // indirect
+	github.com/aws/aws-sdk-go-v2/service/internal/presigned-url v1.13.17 // indirect
+	github.com/aws/aws-sdk-go-v2/service/internal/s3shared v1.19.17 // indirect
+	github.com/aws/aws-sdk-go-v2/service/signin v1.0.5 // indirect
+	github.com/aws/aws-sdk-go-v2/service/sso v1.30.9 // indirect
+	github.com/aws/aws-sdk-go-v2/service/ssooidc v1.35.13 // indirect
+	github.com/aws/aws-sdk-go-v2/service/sts v1.41.6 // indirect
 )

go.sum

@@ -1,43 +1,45 @@
-github.com/aws/aws-sdk-go-v2 v1.30.4 h1:frhcagrVNrzmT95RJImMHgabt99vkXGslubDaDagTk8=
-github.com/aws/aws-sdk-go-v2 v1.30.4/go.mod h1:CT+ZPWXbYrci8chcARI3OmI/qgd+f6WtuLOoaIA8PR0=
-github.com/aws/aws-sdk-go-v2/aws/protocol/eventstream v1.6.4 h1:70PVAiL15/aBMh5LThwgXdSQorVr91L127ttckI9QQU=
-github.com/aws/aws-sdk-go-v2/aws/protocol/eventstream v1.6.4/go.mod h1:/MQxMqci8tlqDH+pjmoLu1i0tbWCUP1hhyMRuFxpQCw=
-github.com/aws/aws-sdk-go-v2/config v1.27.31 h1:kxBoRsjhT3pq0cKthgj6RU6bXTm/2SgdoUMyrVw0rAI=
-github.com/aws/aws-sdk-go-v2/config v1.27.31/go.mod h1:z04nZdSWFPaDwK3DdJOG2r+scLQzMYuJeW0CujEm9FM=
-github.com/aws/aws-sdk-go-v2/credentials v1.17.30 h1:aau/oYFtibVovr2rDt8FHlU17BTicFEMAi29V1U+L5Q=
-github.com/aws/aws-sdk-go-v2/credentials v1.17.30/go.mod h1:BPJ/yXV92ZVq6G8uYvbU0gSl8q94UB63nMT5ctNO38g=
-github.com/aws/aws-sdk-go-v2/feature/ec2/imds v1.16.12 h1:yjwoSyDZF8Jth+mUk5lSPJCkMC0lMy6FaCD51jm6ayE=
-github.com/aws/aws-sdk-go-v2/feature/ec2/imds v1.16.12/go.mod h1:fuR57fAgMk7ot3WcNQfb6rSEn+SUffl7ri+aa8uKysI=
-github.com/aws/aws-sdk-go-v2/internal/configsources v1.3.16 h1:TNyt/+X43KJ9IJJMjKfa3bNTiZbUP7DeCxfbTROESwY=
-github.com/aws/aws-sdk-go-v2/internal/configsources v1.3.16/go.mod h1:2DwJF39FlNAUiX5pAc0UNeiz16lK2t7IaFcm0LFHEgc=
-github.com/aws/aws-sdk-go-v2/internal/endpoints/v2 v2.6.16 h1:jYfy8UPmd+6kJW5YhY0L1/KftReOGxI/4NtVSTh9O/I=
-github.com/aws/aws-sdk-go-v2/internal/endpoints/v2 v2.6.16/go.mod h1:7ZfEPZxkW42Afq4uQB8H2E2e6ebh6mXTueEpYzjCzcs=
-github.com/aws/aws-sdk-go-v2/internal/ini v1.8.1 h1:VaRN3TlFdd6KxX1x3ILT5ynH6HvKgqdiXoTxAF4HQcQ=
-github.com/aws/aws-sdk-go-v2/internal/ini v1.8.1/go.mod h1:FbtygfRFze9usAadmnGJNc8KsP346kEe+y2/oyhGAGc=
-github.com/aws/aws-sdk-go-v2/internal/v4a v1.3.16 h1:mimdLQkIX1zr8GIPY1ZtALdBQGxcASiBd2MOp8m/dMc=
-github.com/aws/aws-sdk-go-v2/internal/v4a v1.3.16/go.mod h1:YHk6owoSwrIsok+cAH9PENCOGoH5PU2EllX4vLtSrsY=
-github.com/aws/aws-sdk-go-v2/service/internal/accept-encoding v1.11.4 h1:KypMCbLPPHEmf9DgMGw51jMj77VfGPAN2Kv4cfhlfgI=
-github.com/aws/aws-sdk-go-v2/service/internal/accept-encoding v1.11.4/go.mod h1:Vz1JQXliGcQktFTN/LN6uGppAIRoLBR2bMvIMP0gOjc=
-github.com/aws/aws-sdk-go-v2/service/internal/checksum v1.3.18 h1:GckUnpm4EJOAio1c8o25a+b3lVfwVzC9gnSBqiiNmZM=
-github.com/aws/aws-sdk-go-v2/service/internal/checksum v1.3.18/go.mod h1:Br6+bxfG33Dk3ynmkhsW2Z/t9D4+lRqdLDNCKi85w0U=
-github.com/aws/aws-sdk-go-v2/service/internal/presigned-url v1.11.18 h1:tJ5RnkHCiSH0jyd6gROjlJtNwov0eGYNz8s8nFcR0jQ=
-github.com/aws/aws-sdk-go-v2/service/internal/presigned-url v1.11.18/go.mod h1:++NHzT+nAF7ZPrHPsA+ENvsXkOO8wEu+C6RXltAG4/c=
-github.com/aws/aws-sdk-go-v2/service/internal/s3shared v1.17.16 h1:jg16PhLPUiHIj8zYIW6bqzeQSuHVEiWnGA0Brz5Xv2I=
-github.com/aws/aws-sdk-go-v2/service/internal/s3shared v1.17.16/go.mod h1:Uyk1zE1VVdsHSU7096h/rwnXDzOzYQVl+FNPhPw7ShY=
-github.com/aws/aws-sdk-go-v2/service/s3 v1.61.0 h1:Wb544Wh+xfSXqJ/j3R4aX9wrKUoZsJNmilBYZb3mKQ4=
-github.com/aws/aws-sdk-go-v2/service/s3 v1.61.0/go.mod h1:BSPI0EfnYUuNHPS0uqIo5VrRwzie+Fp+YhQOUs16sKI=
-github.com/aws/aws-sdk-go-v2/service/sso v1.22.5 h1:zCsFCKvbj25i7p1u94imVoO447I/sFv8qq+lGJhRN0c=
-github.com/aws/aws-sdk-go-v2/service/sso v1.22.5/go.mod h1:ZeDX1SnKsVlejeuz41GiajjZpRSWR7/42q/EyA/QEiM=
-github.com/aws/aws-sdk-go-v2/service/ssooidc v1.26.5 h1:SKvPgvdvmiTWoi0GAJ7AsJfOz3ngVkD/ERbs5pUnHNI=
-github.com/aws/aws-sdk-go-v2/service/ssooidc v1.26.5/go.mod h1:20sz31hv/WsPa3HhU3hfrIet2kxM4Pe0r20eBZ20Tac=
-github.com/aws/aws-sdk-go-v2/service/sts v1.30.5 h1:OMsEmCyz2i89XwRwPouAJvhj81wINh+4UK+k/0Yo/q8=
-github.com/aws/aws-sdk-go-v2/service/sts v1.30.5/go.mod h1:vmSqFK+BVIwVpDAGZB3CoCXHzurt4qBE8lf+I/kRTh0=
-github.com/aws/smithy-go v1.20.4 h1:2HK1zBdPgRbjFOHlfeQZfpC4r72MOb9bZkiFwggKO+4=
-github.com/aws/smithy-go v1.20.4/go.mod h1:irrKGvNn1InZwb2d7fkIRNucdfwR8R+Ts3wxYa/cJHg=
+github.com/aws/aws-sdk-go-v2 v1.41.1 h1:ABlyEARCDLN034NhxlRUSZr4l71mh+T5KAeGh6cerhU=
+github.com/aws/aws-sdk-go-v2 v1.41.1/go.mod h1:MayyLB8y+buD9hZqkCW3kX1AKq07Y5pXxtgB+rRFhz0=
+github.com/aws/aws-sdk-go-v2/aws/protocol/eventstream v1.7.4 h1:489krEF9xIGkOaaX3CE/Be2uWjiXrkCH6gUX+bZA/BU=
+github.com/aws/aws-sdk-go-v2/aws/protocol/eventstream v1.7.4/go.mod h1:IOAPF6oT9KCsceNTvvYMNHy0+kMF8akOjeDvPENWxp4=
+github.com/aws/aws-sdk-go-v2/config v1.32.7 h1:vxUyWGUwmkQ2g19n7JY/9YL8MfAIl7bTesIUykECXmY=
+github.com/aws/aws-sdk-go-v2/config v1.32.7/go.mod h1:2/Qm5vKUU/r7Y+zUk/Ptt2MDAEKAfUtKc1+3U1Mo3oY=
+github.com/aws/aws-sdk-go-v2/credentials v1.19.7 h1:tHK47VqqtJxOymRrNtUXN5SP/zUTvZKeLx4tH6PGQc8=
+github.com/aws/aws-sdk-go-v2/credentials v1.19.7/go.mod h1:qOZk8sPDrxhf+4Wf4oT2urYJrYt3RejHSzgAquYeppw=
+github.com/aws/aws-sdk-go-v2/feature/ec2/imds v1.18.17 h1:I0GyV8wiYrP8XpA70g1HBcQO1JlQxCMTW9npl5UbDHY=
+github.com/aws/aws-sdk-go-v2/feature/ec2/imds v1.18.17/go.mod h1:tyw7BOl5bBe/oqvoIeECFJjMdzXoa/dfVz3QQ5lgHGA=
+github.com/aws/aws-sdk-go-v2/internal/configsources v1.4.17 h1:xOLELNKGp2vsiteLsvLPwxC+mYmO6OZ8PYgiuPJzF8U=
+github.com/aws/aws-sdk-go-v2/internal/configsources v1.4.17/go.mod h1:5M5CI3D12dNOtH3/mk6minaRwI2/37ifCURZISxA/IQ=
+github.com/aws/aws-sdk-go-v2/internal/endpoints/v2 v2.7.17 h1:WWLqlh79iO48yLkj1v3ISRNiv+3KdQoZ6JWyfcsyQik=
+github.com/aws/aws-sdk-go-v2/internal/endpoints/v2 v2.7.17/go.mod h1:EhG22vHRrvF8oXSTYStZhJc1aUgKtnJe+aOiFEV90cM=
+github.com/aws/aws-sdk-go-v2/internal/ini v1.8.4 h1:WKuaxf++XKWlHWu9ECbMlha8WOEGm0OUEZqm4K/Gcfk=
+github.com/aws/aws-sdk-go-v2/internal/ini v1.8.4/go.mod h1:ZWy7j6v1vWGmPReu0iSGvRiise4YI5SkR3OHKTZ6Wuc=
+github.com/aws/aws-sdk-go-v2/internal/v4a v1.4.17 h1:JqcdRG//czea7Ppjb+g/n4o8i/R50aTBHkA7vu0lK+k=
+github.com/aws/aws-sdk-go-v2/internal/v4a v1.4.17/go.mod h1:CO+WeGmIdj/MlPel2KwID9Gt7CNq4M65HUfBW97liM0=
+github.com/aws/aws-sdk-go-v2/service/internal/accept-encoding v1.13.4 h1:0ryTNEdJbzUCEWkVXEXoqlXV72J5keC1GvILMOuD00E=
+github.com/aws/aws-sdk-go-v2/service/internal/accept-encoding v1.13.4/go.mod h1:HQ4qwNZh32C3CBeO6iJLQlgtMzqeG17ziAA/3KDJFow=
+github.com/aws/aws-sdk-go-v2/service/internal/checksum v1.9.8 h1:Z5EiPIzXKewUQK0QTMkutjiaPVeVYXX7KIqhXu/0fXs=
+github.com/aws/aws-sdk-go-v2/service/internal/checksum v1.9.8/go.mod h1:FsTpJtvC4U1fyDXk7c71XoDv3HlRm8V3NiYLeYLh5YE=
+github.com/aws/aws-sdk-go-v2/service/internal/presigned-url v1.13.17 h1:RuNSMoozM8oXlgLG/n6WLaFGoea7/CddrCfIiSA+xdY=
+github.com/aws/aws-sdk-go-v2/service/internal/presigned-url v1.13.17/go.mod h1:F2xxQ9TZz5gDWsclCtPQscGpP0VUOc8RqgFM3vDENmU=
+github.com/aws/aws-sdk-go-v2/service/internal/s3shared v1.19.17 h1:bGeHBsGZx0Dvu/eJC0Lh9adJa3M1xREcndxLNZlve2U=
+github.com/aws/aws-sdk-go-v2/service/internal/s3shared v1.19.17/go.mod h1:dcW24lbU0CzHusTE8LLHhRLI42ejmINN8Lcr22bwh/g=
+github.com/aws/aws-sdk-go-v2/service/s3 v1.96.0 h1:oeu8VPlOre74lBA/PMhxa5vewaMIMmILM+RraSyB8KA=
+github.com/aws/aws-sdk-go-v2/service/s3 v1.96.0/go.mod h1:5jggDlZ2CLQhwJBiZJb4vfk4f0GxWdEDruWKEJ1xOdo=
+github.com/aws/aws-sdk-go-v2/service/signin v1.0.5 h1:VrhDvQib/i0lxvr3zqlUwLwJP4fpmpyD9wYG1vfSu+Y=
+github.com/aws/aws-sdk-go-v2/service/signin v1.0.5/go.mod h1:k029+U8SY30/3/ras4G/Fnv/b88N4mAfliNn08Dem4M=
+github.com/aws/aws-sdk-go-v2/service/sso v1.30.9 h1:v6EiMvhEYBoHABfbGB4alOYmCIrcgyPPiBE1wZAEbqk=
+github.com/aws/aws-sdk-go-v2/service/sso v1.30.9/go.mod h1:yifAsgBxgJWn3ggx70A3urX2AN49Y5sJTD1UQFlfqBw=
+github.com/aws/aws-sdk-go-v2/service/ssooidc v1.35.13 h1:gd84Omyu9JLriJVCbGApcLzVR3XtmC4ZDPcAI6Ftvds=
+github.com/aws/aws-sdk-go-v2/service/ssooidc v1.35.13/go.mod h1:sTGThjphYE4Ohw8vJiRStAcu3rbjtXRsdNB0TvZ5wwo=
+github.com/aws/aws-sdk-go-v2/service/sts v1.41.6 h1:5fFjR/ToSOzB2OQ/XqWpZBmNvmP/pJ1jOWYlFDJTjRQ=
+github.com/aws/aws-sdk-go-v2/service/sts v1.41.6/go.mod h1:qgFDZQSD/Kys7nJnVqYlWKnh0SSdMjAi0uSwON4wgYQ=
+github.com/aws/smithy-go v1.24.0 h1:LpilSUItNPFr1eY85RYgTIg5eIEPtvFbskaFcmmIUnk=
+github.com/aws/smithy-go v1.24.0/go.mod h1:LEj2LM3rBRQJxPZTB4KuzZkaZYnZPnvgIhb4pu07mx0=
 github.com/pkg/term v1.1.0 h1:xIAAdCMh3QIAy+5FrE8Ad8XoDhEU4ufwbaSozViP9kk=
 github.com/pkg/term v1.1.0/go.mod h1:E25nymQcrSllhX42Ok8MRm1+hyBdHY0dCeiKZ9jpNGw=
 github.com/stefansundin/go-zflag v1.1.1 h1:XabhzWS588bVvV1z1UctSa6i8zHkXc5W9otqtnDSHw8=
 github.com/stefansundin/go-zflag v1.1.1/go.mod h1:HXX5rABl1AoTcZ2jw+CqJ7R8irczaLquGNZlFabZooc=
 golang.org/x/sys v0.0.0-20200909081042-eff7692f9009/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
-golang.org/x/sys v0.24.0 h1:Twjiwq9dn6R1fQcyiK+wQyHWfaz/BJB+YIpzU/Cv3Xg=
-golang.org/x/sys v0.24.0/go.mod h1:/VUhepiaJMQUp4+oa/7Zr1D23ma6VTLIYjOOTFZPUcA=
+golang.org/x/sys v0.41.0 h1:Ivj+2Cp/ylzLiEU89QhWblYnOE9zerudt9Ftecq2C6k=
+golang.org/x/sys v0.41.0/go.mod h1:OgkHotnGiDImocRcuBABYBEXf8A9a87e/uXjp9XT3ks=

main.go

@@ -17,6 +17,7 @@ import (
 	"os"
 	"os/signal"
 	"runtime"
+	"runtime/secret"
 	"sort"
 	"strings"
 	"time"
@@ -155,27 +156,30 @@ func run() (int, error) {
 		fmt.Fprintln(os.Stderr, "Warning: MFA duration can not exceed 12 hours.")
 	}
 	if mfaSecretFlag {
-		fmt.Fprintln(os.Stderr, "Read more about the --mfa-secret feature here: https://github.com/stefansundin/shrimp/discussions/3")
-		secret, ok := os.LookupEnv("AWS_MFA_SECRET")
-		if ok {
-			fmt.Fprintln(os.Stderr, "MFA secret read from AWS_MFA_SECRET.")
-		} else {
-			fmt.Fprint(os.Stderr, "MFA secret: ")
-			_, err := fmt.Scanln(&secret)
-			fmt.Fprint(os.Stderr, "\033[1A\033[2K") // erase the line
-			if err != nil {
-				return 1, err
-			}
-		}
-		fmt.Fprintln(os.Stderr)
-		// Normalize secret
-		secret = strings.TrimSpace(secret)
-		if n := len(secret) % 8; n != 0 {
-			secret = secret + strings.Repeat("=", 8-n)
-		}
-		secret = strings.ToUpper(secret)
 		var err error
-		mfaSecret, err = base32.StdEncoding.DecodeString(secret)
+		secret.Do(func() {
+			fmt.Fprintln(os.Stderr, "Read more about the --mfa-secret feature here: https://github.com/stefansundin/shrimp/discussions/3")
+			s, ok := os.LookupEnv("AWS_MFA_SECRET")
+			if ok {
+				os.Unsetenv("AWS_MFA_SECRET")
+				fmt.Fprintln(os.Stderr, "MFA secret read from AWS_MFA_SECRET.")
+			} else {
+				fmt.Fprint(os.Stderr, "MFA secret: ")
+				_, err := fmt.Scanln(&s)
+				fmt.Fprint(os.Stderr, "\033[1A\033[2K") // erase the line
+				if err != nil {
+					return
+				}
+			}
+			fmt.Fprintln(os.Stderr)
+			// Normalize secret
+			s = strings.TrimSpace(s)
+			if n := len(s) % 8; n != 0 {
+				s = s + strings.Repeat("=", 8-n)
+			}
+			s = strings.ToUpper(s)
+			mfaSecret, err = base32.StdEncoding.DecodeString(s)
+		})
 		if err != nil {
 			return 1, errors.New("Error: Invalid MFA secret.")
 		}

utils.go

@@ -14,6 +14,7 @@ import (
 	"math"
 	"os"
 	"path/filepath"
+	"runtime/secret"
 	"strconv"
 	"strings"
 	"time"
@@ -312,21 +313,27 @@ func isSmithyErrorCode(err error, code int) bool {
 }
 
 func generateOTP(secretBytes []byte, counter uint64, hashAlg func() hash.Hash, digits int) (string, error) {
-	mac := hmac.New(hashAlg, secretBytes)
-	buf := make([]byte, 8)
-	binary.BigEndian.PutUint64(buf, counter)
-	_, err := mac.Write(buf)
-	if err != nil {
-		return "", err
-	}
-	sum := mac.Sum(nil)
-	offset := sum[len(sum)-1] & 0xf
-	value := int64(((int(sum[offset]) & 0x7f) << 24) |
-		((int(sum[offset+1] & 0xff)) << 16) |
-		((int(sum[offset+2] & 0xff)) << 8) |
-		(int(sum[offset+3]) & 0xff))
-	mod := int32(value % int64(math.Pow10(digits)))
-	return fmt.Sprintf(fmt.Sprintf("%%0%dd", digits), mod), nil
+	var otp string
+	var err error
+
+	secret.Do(func() {
+		mac := hmac.New(hashAlg, secretBytes)
+		buf := make([]byte, 8)
+		binary.BigEndian.PutUint64(buf, counter)
+		_, err = mac.Write(buf)
+		if err != nil {
+			return
+		}
+		sum := mac.Sum(nil)
+		offset := sum[len(sum)-1] & 0xf
+		value := int64(((int(sum[offset]) & 0x7f) << 24) |
+			((int(sum[offset+1] & 0xff)) << 16) |
+			((int(sum[offset+2] & 0xff)) << 8) |
+			(int(sum[offset+3]) & 0xff))
+		mod := int32(value % int64(math.Pow10(digits)))
+		otp = fmt.Sprintf(fmt.Sprintf("%%0%dd", digits), mod)
+	})
+	return otp, err
 }
 
 //lint:file-ignore ST1005 Some errors are printed as diagnostic output and need proper punctuation