cyberdot · Sep 19, 2026 · Sep 19, 2026 · Sep 19, 2026
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
@@ -44,6 +44,7 @@ jobs:
           for project in \
             src/CyberDot.Encoding.Base65536/CyberDot.Encoding.Base65536.csproj \
             src/CyberDot.Encoding.Base2048/CyberDot.Encoding.Base2048.csproj \
-            src/CyberDot.Encoding.Base84/CyberDot.Encoding.Base84.csproj; do
+            src/CyberDot.Encoding.Base84/CyberDot.Encoding.Base84.csproj \
+            src/CyberDot.Encoding.Base91/CyberDot.Encoding.Base91.csproj; do
             dotnet pack "$project" --configuration Debug --no-build --output "${{ runner.temp }}/nupkg-verify"
           done
diff --git a/.github/workflows/release-base91.yml b/.github/workflows/release-base91.yml
@@ -0,0 +1,22 @@
+name: Release CyberDot.Encoding.Base91
+
+# Manually-triggered release for the Base91 package. See _release-package.yml for
+# what this actually does.
+
+on:
+  workflow_dispatch:
+    inputs:
+      version:
+        description: 'Release version override (e.g. 1.2.0). Leave empty to use the version in the csproj.'
+        required: false
+        type: string
+
+jobs:
+  release:
+    permissions:
+      contents: write
+    uses: ./.github/workflows/_release-package.yml
+    with:
+      project-path: src/CyberDot.Encoding.Base91/CyberDot.Encoding.Base91.csproj
+      tag-prefix: base91
+      version: ${{ inputs.version }}
diff --git a/CyberDot.Encoding.sln b/CyberDot.Encoding.sln
@@ -21,6 +21,12 @@ Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "CyberDot.Encoding.Base84.Te
 EndProject
 Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "Base84", "Base84", "{4C6BEADC-6B8C-425C-9EC4-559597A7915C}"
 EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "CyberDot.Encoding.Base91", "src\CyberDot.Encoding.Base91\CyberDot.Encoding.Base91.csproj", "{7454C92C-8908-4EE6-8A87-FED28AA7F6BE}"
+EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "CyberDot.Encoding.Base91.Tests", "src\CyberDot.Encoding.Base91.Tests\CyberDot.Encoding.Base91.Tests.csproj", "{D14013B6-8905-46BF-8B40-61AE19476DF0}"
+EndProject
+Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "Base91", "Base91", "{6BF31631-01AB-4D7B-A0D0-512C70365327}"
+EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
 		Debug|Any CPU = Debug|Any CPU
@@ -51,6 +57,14 @@ Global
 		{3762FA86-26D7-4037-B7CF-E5CA94F03D8B}.Debug|Any CPU.Build.0 = Debug|Any CPU
 		{3762FA86-26D7-4037-B7CF-E5CA94F03D8B}.Release|Any CPU.ActiveCfg = Release|Any CPU
 		{3762FA86-26D7-4037-B7CF-E5CA94F03D8B}.Release|Any CPU.Build.0 = Release|Any CPU
+		{7454C92C-8908-4EE6-8A87-FED28AA7F6BE}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{7454C92C-8908-4EE6-8A87-FED28AA7F6BE}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{7454C92C-8908-4EE6-8A87-FED28AA7F6BE}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{7454C92C-8908-4EE6-8A87-FED28AA7F6BE}.Release|Any CPU.Build.0 = Release|Any CPU
+		{D14013B6-8905-46BF-8B40-61AE19476DF0}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{D14013B6-8905-46BF-8B40-61AE19476DF0}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{D14013B6-8905-46BF-8B40-61AE19476DF0}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{D14013B6-8905-46BF-8B40-61AE19476DF0}.Release|Any CPU.Build.0 = Release|Any CPU
 	EndGlobalSection
 	GlobalSection(SolutionProperties) = preSolution
 		HideSolutionNode = FALSE
@@ -62,6 +76,8 @@ Global
 		{287EE26D-CAE7-4AA3-93C5-4A6B2C8A9D05} = {7F2B409E-7822-462C-8152-0DE5BEE42CAD}
 		{9551EC7C-8F4A-462A-AABC-50D1FD1E60B5} = {4C6BEADC-6B8C-425C-9EC4-559597A7915C}
 		{3762FA86-26D7-4037-B7CF-E5CA94F03D8B} = {4C6BEADC-6B8C-425C-9EC4-559597A7915C}
+		{7454C92C-8908-4EE6-8A87-FED28AA7F6BE} = {6BF31631-01AB-4D7B-A0D0-512C70365327}
+		{D14013B6-8905-46BF-8B40-61AE19476DF0} = {6BF31631-01AB-4D7B-A0D0-512C70365327}
 	EndGlobalSection
 	GlobalSection(ExtensibilityGlobals) = postSolution
 		SolutionGuid = {9D040A49-47AE-4503-B1C2-7D3E82AF66AB}

diff --git a/src/CyberDot.Encoding.Base2048.Tests/CyberDot.Encoding.Base2048.Tests.csproj b/src/CyberDot.Encoding.Base2048.Tests/CyberDot.Encoding.Base2048.Tests.csproj
@@ -8,8 +8,8 @@
 	</PropertyGroup>
 
 	<ItemGroup>
-		<PackageReference Include="AwesomeAssertions" Version="9.5.0" />
-		<PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.9.0" />
+		<PackageReference Include="AwesomeAssertions" Version="9.6.0" />
+		<PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.10.1" />
 		<PackageReference Include="xunit" Version="2.9.3" />
 		<PackageReference Include="xunit.runner.visualstudio" Version="4.0.0">
 			<PrivateAssets>all</PrivateAssets>

diff --git a/src/CyberDot.Encoding.Base65536.Tests/CyberDot.Encoding.Base65536.Tests.csproj b/src/CyberDot.Encoding.Base65536.Tests/CyberDot.Encoding.Base65536.Tests.csproj
@@ -5,8 +5,8 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="AwesomeAssertions" Version="9.5.0" />
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.9.0" />
+    <PackageReference Include="AwesomeAssertions" Version="9.6.0" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.10.1" />
     <PackageReference Include="xunit" Version="2.9.3" />
     <PackageReference Include="xunit.runner.visualstudio" Version="4.0.0">
       <PrivateAssets>all</PrivateAssets>

diff --git a/src/CyberDot.Encoding.Base84.Tests/CyberDot.Encoding.Base84.Tests.csproj b/src/CyberDot.Encoding.Base84.Tests/CyberDot.Encoding.Base84.Tests.csproj
@@ -8,11 +8,17 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="AwesomeAssertions" Version="9.5.0" />
-    <PackageReference Include="coverlet.collector" Version="6.0.4" />
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.14.1" />
+    <PackageReference Include="AwesomeAssertions" Version="9.6.0" />
+    <PackageReference Include="coverlet.collector" Version="10.0.1">
+      <PrivateAssets>all</PrivateAssets>
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+    </PackageReference>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.10.1" />
     <PackageReference Include="xunit" Version="2.9.3" />
-    <PackageReference Include="xunit.runner.visualstudio" Version="3.1.4" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="4.0.0">
+      <PrivateAssets>all</PrivateAssets>
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+    </PackageReference>
   </ItemGroup>
 
   <ItemGroup>

diff --git a/src/CyberDot.Encoding.Base91.Tests/Base91Tests.cs b/src/CyberDot.Encoding.Base91.Tests/Base91Tests.cs
@@ -0,0 +1,182 @@
+using AwesomeAssertions;
+
+namespace CyberDot.Encoding.Base91.Tests
+{
+    public class Base91Tests
+    {
+        // Vectors below were generated from a direct Python port of the reference basE91
+        // algorithm (refimplementation.txt); "test" -> "fPNKd" also matches the canonical
+        // basE91 example from Joachim Henke's original implementation.
+        [Fact]
+        public void Should_encode_and_decode_known_vectors()
+        {
+            AssertVector("", "");
+            AssertVector("M", "=A");
+            AssertVector("Ma", "8DD");
+            AssertVector("Man", "8D$J");
+            AssertVector("Man ", "8D$JI");
+            AssertVector("test", "fPNKd");
+            AssertVector("Hello, World!", ">OwJh>}AQ;r@@Y?F");
+            AssertVector("The quick brown fox jumps over the lazy dog.",
+                "nX^Iz?T1s!2t:aRn#o>vf>6C9#`##mlLK#_1:Wzv;RG!,a%q3Lc=5gA");
+        }
+
+        [Fact]
+        public void Should_encode_and_decode_runs_of_zero_bytes()
+        {
+            AssertVector(new byte[] { 0x00 }, "AA");
+            AssertVector(new byte[] { 0x00, 0x00 }, "AAA");
+            AssertVector(new byte[] { 0x00, 0x00, 0x00 }, "AAAA");
+            AssertVector(new byte[] { 0x00, 0x00, 0x00, 0x00 }, "AAAAA");
+            AssertVector(new byte[] { 0x00, 0x00, 0x00, 0x00, 0x00 }, "AAAAAA");
+        }
+
+        [Fact]
+        public void Should_encode_and_decode_runs_of_ff_bytes()
+        {
+            AssertVector(new byte[] { 0xff }, "/C");
+            AssertVector(new byte[] { 0xff, 0xff }, "B\"H");
+            AssertVector(new byte[] { 0xff, 0xff, 0xff }, "B\"tW");
+            AssertVector(new byte[] { 0xff, 0xff, 0xff, 0xff }, "B\"B\"#");
+            AssertVector(new byte[] { 0xff, 0xff, 0xff, 0xff, 0xff }, "B\"B\"B\"B");
+        }
+
+        [Fact]
+        public void Should_encode_and_decode_mixed_bytes()
+        {
+            AssertVector(new byte[] { 0x00, 0xe0, 0xff, 0x01 }, "C\"tWA");
+            AssertVector(
+                new byte[] { 0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f },
+                ":C#(:C?hVB$MSiVEwndB");
+        }
+
+        [Theory]
+        [InlineData("AA ")]
+        [InlineData("AA'")]
+        [InlineData("AA-")]
+        [InlineData("AA\\")]
+        [InlineData("AA\x00")]
+        [InlineData("AA\x7f")]
+        public void Should_throw_on_unrecognised_character(string input)
+        {
+            var act = () => Base91.Decode(input);
+
+            act.Should().Throw<ArgumentException>();
+        }
+
+        [Fact]
+        public void Should_throw_on_null_input()
+        {
+            var encodeAct = () => Base91.Encode(null);
+            var decodeAct = () => Base91.Decode(null);
+
+            encodeAct.Should().Throw<ArgumentNullException>();
+            decodeAct.Should().Throw<ArgumentNullException>();
+        }
+
+        [Fact]
+        public void Should_round_trip_all_single_bytes()
+        {
+            for (var b = 0; b <= 255; b++)
+            {
+                var data = new[] { (byte)b };
+
+                var encoded = Base91.Encode(data);
+                Base91.Decode(encoded).Should().BeEquivalentTo(data);
+            }
+        }
+
+        [Fact]
+        public void Should_round_trip_all_byte_pairs()
+        {
+            for (var a = 0; a <= 255; a++)
+            {
+                for (var b = 0; b <= 255; b++)
+                {
+                    var data = new[] { (byte)a, (byte)b };
+
+                    var encoded = Base91.Encode(data);
+                    Base91.Decode(encoded).Should().BeEquivalentTo(data);
+                }
+            }
+        }
+
+        [Theory]
+        [InlineData(0)]
+        [InlineData(1)]
+        [InlineData(2)]
+        [InlineData(3)]
+        [InlineData(4)]
+        [InlineData(5)]
+        [InlineData(6)]
+        [InlineData(17)]
+        [InlineData(31)]
+        [InlineData(32)]
+        [InlineData(33)]
+        [InlineData(256)]
+        public void Should_round_trip_random_data(int length)
+        {
+            var data = new byte[length];
+            new Random(length).NextBytes(data);
+
+            var encoded = Base91.Encode(data);
+            Base91.Decode(encoded).Should().BeEquivalentTo(data);
+        }
+
+        [Fact]
+        public void Should_round_trip_many_random_lengths()
+        {
+            var random = new Random(12345);
+            var buffer = new byte[256];
+
+            for (var i = 0; i < 5000; i++)
+            {
+                var len = random.Next(0, buffer.Length + 1);
+                random.NextBytes(buffer);
+                var data = new byte[len];
+                Array.Copy(buffer, data, len);
+
+                var encoded = Base91.Encode(data);
+                Base91.Decode(encoded).Should().BeEquivalentTo(data);
+            }
+        }
+
+        // Unlike Base84/Base2048, Base91 has no fixed-width character group, so decoding
+        // cannot validate padding - any string built purely from alphabet characters decodes
+        // without throwing, even if it was never produced by Encode.
+        [Fact]
+        public void Should_never_throw_when_decoding_any_string_of_alphabet_characters()
+        {
+            var random = new Random(999);
+            var buffer = new char[64];
+
+            for (var i = 0; i < 2000; i++)
+            {
+                var len = random.Next(0, buffer.Length + 1);
+                for (var j = 0; j < len; j++)
+                {
+                    buffer[j] = AlphabetChars[random.Next(AlphabetChars.Length)];
+                }
+
+                var candidate = new string(buffer, 0, len);
+
+                var act = () => Base91.Decode(candidate);
+
+                act.Should().NotThrow();
+            }
+        }
+
+        private const string AlphabetChars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789!#$%&()*+,./:;<=>?@[]^_`{|}~\"";
+
+        private static void AssertVector(string decoded, string encoded)
+        {
+            AssertVector(System.Text.Encoding.UTF8.GetBytes(decoded), encoded);
+        }
+
+        private static void AssertVector(byte[] decoded, string encoded)
+        {
+            Base91.Encode(decoded).Should().Be(encoded);
+            Base91.Decode(encoded).Should().BeEquivalentTo(decoded);
+        }
+    }
+}
diff --git a/src/CyberDot.Encoding.Base91.Tests/CyberDot.Encoding.Base91.Tests.csproj b/src/CyberDot.Encoding.Base91.Tests/CyberDot.Encoding.Base91.Tests.csproj
@@ -0,0 +1,32 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="AwesomeAssertions" Version="9.6.0" />
+    <PackageReference Include="coverlet.collector" Version="10.0.1">
+      <PrivateAssets>all</PrivateAssets>
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+    </PackageReference>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.10.1" />
+    <PackageReference Include="xunit" Version="2.9.3" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="4.0.0">
+      <PrivateAssets>all</PrivateAssets>
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+    </PackageReference>
+  </ItemGroup>
+
+  <ItemGroup>
+    <Using Include="Xunit" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\CyberDot.Encoding.Base91\CyberDot.Encoding.Base91.csproj" />
+  </ItemGroup>
+
+</Project>