diff --git a/src/Cask.sln b/src/Cask.sln
index 1dbb83a..750a177 100644
--- a/src/Cask.sln
+++ b/src/Cask.sln
@@ -9,6 +9,7 @@ Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Cask", "Cask\Cask.csproj",
 EndProject
 Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "Tests", "Tests", "{AA9664D7-21A5-4941-BE8A-D62765F58CE6}"
 	ProjectSection(SolutionItems) = preProject
+		Tests\.editorconfig = Tests\.editorconfig
 		Tests\Directory.Build.props = Tests\Directory.Build.props
 		Tests\Directory.Packages.props = Tests\Directory.Packages.props
 	EndProjectSection
diff --git a/src/Cask/CrossCompanyCorrelatingId.cs b/src/Cask/CrossCompanyCorrelatingId.cs
new file mode 100644
index 0000000..cf89707
--- /dev/null
+++ b/src/Cask/CrossCompanyCorrelatingId.cs
@@ -0,0 +1,93 @@
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using System.Security.Cryptography;
+using System.Text;
+
+using static CommonAnnotatedSecurityKeys.Limits;
+
+namespace CommonAnnotatedSecurityKeys;
+
+/// <summary>
+/// Cross-Company Correlating Id (C3ID) a 12-byte value used to correlate a
+/// high-entropy keys with other data. The canonical textual representation is
+/// base64 encoded and prefixed with "C3ID".
+/// </summary>
+public static class CrossCompanyCorrelatingId
+{
+    /// <summary>
+    /// The size of a C3ID in raw bytes.
+    /// </summary>
+    public const int RawSizeInBytes = 12;
+
+    /// <summary>
+    /// The byte sequence prepended to the input for the first SHA256 hash. It
+    /// is defined as the UTF-8 encoding of "C3ID".
+    /// </summary>
+    private static ReadOnlySpan<byte> Prefix => "C3ID"u8;
+
+    /// <summary>
+    /// The byte sequence prepended to the to the output of the
+    /// base64-encoding. It is defined as the base64-decoding of "C3ID". This
+    /// results in all canonical base64 encoded C3IDs starting with "C3ID".
+    /// </summary>
+    private static ReadOnlySpan<byte> PrefixBase64Decoded => [0x0B, 0x72, 0x03];
+
+    /// <summary>
+    /// Computes the C3ID for the given text in canonical textual form.
+    /// </summary>
+    public static string Compute(string text)
+    {
+        ThrowIfNullOrEmpty(text);
+        Span<byte> bytes = stackalloc byte[PrefixBase64Decoded.Length + RawSizeInBytes];
+        PrefixBase64Decoded.CopyTo(bytes);
+        ComputeRaw(text, bytes[PrefixBase64Decoded.Length..]);
+        return Convert.ToBase64String(bytes);
+    }
+
+    /// <summary>
+    /// Computes the raw C3ID bytes for the given text and writes them to the
+    /// destination span.
+    /// </summary>
+    public static void ComputeRaw(string text, Span<byte> destination)
+    {
+        ThrowIfNull(text);
+        ComputeRaw(text.AsSpan(), destination);
+    }
+
+    /// <summary>
+    /// Computes the raw C3ID bytes for the given UTF-16 encoded text sequence
+    /// and writes them to the destination span.
+    /// </summary>
+    public static void ComputeRaw(ReadOnlySpan<char> text, Span<byte> destination)
+    {
+        ThrowIfEmpty(text);
+        ThrowIfDestinationTooSmall(destination, RawSizeInBytes);
+
+        int byteCount = Encoding.UTF8.GetByteCount(text);
+        Span<byte> textUtf8 = byteCount <= MaxStackAlloc ? stackalloc byte[byteCount] : new byte[byteCount];
+        Encoding.UTF8.GetBytes(text, textUtf8);
+        ComputeRawUtf8(textUtf8, destination);
+    }
+
+    /// <summary>
+    /// Computes the raw C3ID bytes for the given UTF-8 encoded text sequence
+    /// and writes them to the destination span.
+    /// </summary>>
+    public static void ComputeRawUtf8(ReadOnlySpan<byte> textUtf8, Span<byte> destination)
+    {
+        ThrowIfEmpty(textUtf8);
+        ThrowIfDestinationTooSmall(destination, RawSizeInBytes);
+
+        // Produce input for second hash: "C3ID"u8 + SHA256(text)
+        Span<byte> input = stackalloc byte[Prefix.Length + SHA256.HashSizeInBytes];
+        Prefix.CopyTo(input);
+        SHA256.HashData(textUtf8, input[Prefix.Length..]);
+
+        // Perform second hash, truncate, and copy to destination.
+        Span<byte> sha = stackalloc byte[SHA256.HashSizeInBytes];
+        SHA256.HashData(input, sha);
+        sha[..RawSizeInBytes].CopyTo(destination);
+    }
+}
+
diff --git a/src/Cask/Helpers.cs b/src/Cask/Helpers.cs
index 34dccd2..2eabc3c 100644
--- a/src/Cask/Helpers.cs
+++ b/src/Cask/Helpers.cs
@@ -111,6 +111,14 @@ public static void ThrowIfDestinationTooSmall<T>(Span<T> destination, int requir
         }
     }
 
+    public static void ThrowIfEmpty<T>(ReadOnlySpan<T> value, [CallerArgumentExpression(nameof(value))] string? paramName = null)
+    {
+        if (value.IsEmpty)
+        {
+            ThrowEmpty(paramName);
+        }
+    }
+
     [DoesNotReturn]
     private static void ThrowDefault(string? paramName)
     {
@@ -123,4 +131,9 @@ private static void ThrowDestinationTooSmall(string? paramName)
         throw new ArgumentException("Destination buffer is too small.", paramName);
     }
 
+    [DoesNotReturn]
+    private static void ThrowEmpty(string? paramName)
+    {
+        throw new ArgumentException("Value cannot be empty.", paramName);
+    }
 }
diff --git a/src/Cask/Polyfill.GlobalUsings.cs b/src/Cask/Polyfill.GlobalUsings.cs
index e224ab0..072e89b 100644
--- a/src/Cask/Polyfill.GlobalUsings.cs
+++ b/src/Cask/Polyfill.GlobalUsings.cs
@@ -8,6 +8,7 @@
 
 global using static Polyfill.ArgumentValidation;
 
+global using Convert = Polyfill.Convert;
 global using HMACSHA256 = Polyfill.HMACSHA256;
 global using RandomNumberGenerator = Polyfill.RandomNumberGenerator;
 global using SHA256 = Polyfill.SHA256;
diff --git a/src/Cask/Polyfill.cs b/src/Cask/Polyfill.cs
index aafabba..44bff92 100644
--- a/src/Cask/Polyfill.cs
+++ b/src/Cask/Polyfill.cs
@@ -60,6 +60,7 @@
 using System.Text;
 using System.Text.RegularExpressions;
 
+using Bcl_Convert = System.Convert;
 using Bcl_HMACSHA256 = System.Security.Cryptography.HMACSHA256;
 using Bcl_SHA256 = System.Security.Cryptography.SHA256;
 
@@ -69,6 +70,11 @@ internal static class Extensions
     {
         public static unsafe string GetString(this Encoding encoding, ReadOnlySpan<byte> bytes)
         {
+            if (bytes.Length == 0)
+            {
+                return string.Empty;
+            }
+
             fixed (byte* ptr = bytes)
             {
                 return encoding.GetString(ptr, bytes.Length);
@@ -77,6 +83,11 @@ public static unsafe string GetString(this Encoding encoding, ReadOnlySpan<byte>
 
         public static unsafe int GetByteCount(this Encoding encoding, ReadOnlySpan<char> chars)
         {
+            if (chars.Length == 0)
+            {
+                return 0;
+            }
+
             fixed (char* ptr = chars)
             {
                 return encoding.GetByteCount(ptr, chars.Length);
@@ -85,6 +96,11 @@ public static unsafe int GetByteCount(this Encoding encoding, ReadOnlySpan<char>
 
         public static unsafe int GetBytes(this Encoding encoding, ReadOnlySpan<char> chars, Span<byte> bytes)
         {
+            if (chars.Length == 0)
+            {
+                return 0;
+            }
+
             fixed (char* charPtr = chars)
             fixed (byte* bytePtr = bytes)
             {
@@ -103,6 +119,14 @@ public static void ThrowIfNull([NotNull] object? argument, [CallerArgumentExpres
             }
         }
 
+        public static void ThrowIfNullOrEmpty([NotNull] string? argument, [CallerArgumentExpression(nameof(argument))] string? paramName = null)
+        {
+            if (string.IsNullOrEmpty(argument))
+            {
+                ThrowNullOrEmpty(argument, paramName);
+            }
+        }
+
         public static void ThrowIfGreaterThan(int value, int max, [CallerArgumentExpression(nameof(value))] string? paramName = null)
         {
             if (value > max)
@@ -136,6 +160,26 @@ private static void ThrowLessThan(int value, int min, string? paramName)
         {
             throw new ArgumentOutOfRangeException(paramName, value, $"Value must be greater than or equal to {min}.");
         }
+
+        [DoesNotReturn]
+        private static void ThrowNullOrEmpty(string? argument, string? paramName)
+        {
+            ThrowIfNull(argument, paramName);
+            throw new ArgumentException("Value cannot be empty.", paramName);
+        }
+    }
+
+    internal static class Convert
+    {
+        public static string ToBase64String(ReadOnlySpan<byte> bytes)
+        {
+            return Bcl_Convert.ToBase64String(bytes.ToArray());
+        }
+
+        public static byte[] FromBase64String(string base64)
+        {
+            return Bcl_Convert.FromBase64String(base64);
+        }
     }
 
     internal static class RandomNumberGenerator
@@ -213,6 +257,13 @@ public static int HashData(ReadOnlySpan<byte> source, Span<byte> destination)
             Hash.Compute(sha, source, destination);
             return HashSizeInBytes;
         }
+
+        public static byte[] HashData(ReadOnlySpan<byte> source)
+        {
+            byte[] hash = new byte[HashSizeInBytes];
+            HashData(source, hash);
+            return hash;
+        }
     }
 }
 
diff --git a/src/Tests/.editorconfig b/src/Tests/.editorconfig
index bee9642..4e47142 100644
--- a/src/Tests/.editorconfig
+++ b/src/Tests/.editorconfig
@@ -15,3 +15,6 @@ dotnet_diagnostic.CA1707.severity = silent
 
 # CA1822: Mark members as static
 dotnet_diagnostic.CA1822.severity = silent
+
+# CA1062: Validate arguments of public methods
+dotnet_diagnostic.CA1062.severity = silent
diff --git a/src/Tests/Cask.Tests/CrossCompanyCorrelatingIdTests.cs b/src/Tests/Cask.Tests/CrossCompanyCorrelatingIdTests.cs
new file mode 100644
index 0000000..55e2c40
--- /dev/null
+++ b/src/Tests/Cask.Tests/CrossCompanyCorrelatingIdTests.cs
@@ -0,0 +1,121 @@
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using System.Security.Cryptography;
+using System.Text;
+
+using Xunit;
+
+namespace CommonAnnotatedSecurityKeys.Tests;
+
+public class CrossCompanyCorrelatingIdTests
+{
+    [Theory]
+    [InlineData("Hello world", "C3IDnw4dY6uIibYownZw")]
+    [InlineData("😁", "C3IDF8FaWr4yMPcwOOxM")]
+    [InlineData("y_-KPF3BQb2-VHZeqrp28c6dgiL9y7H9TRJmQ5jJe9OvJQQJTESTBAU4AAB5mIhC", "C3IDKx9aukbRgOnPEyeu")]
+    [InlineData("Kq03wDtdCGWvs3sPgbH84H5MDADIJMZEERRhUN73CaGBJQQJTESTBAU4AADqe9ge", "C3IDO93RBPyuaA6ZRK8+")]
+    public void C3Id_Basic(string text, string expected)
+    {
+        string actual = ComputeC3Id(text);
+        Assert.Equal(expected, actual);
+    }
+
+    [Fact]
+    public void C3Id_LargeText()
+    {
+        string actual = ComputeC3Id(text: new string('x', 300));
+        Assert.Equal("C3IDs+pSKJ1FmRW+7EZk", actual);
+    }
+
+    [Fact]
+    public void C3Id_Null_Throws()
+    {
+        Assert.Throws<ArgumentNullException>("text", () => CrossCompanyCorrelatingId.Compute(null!));
+    }
+
+    [Fact]
+    public void C3Id_Empty_Throws()
+    {
+        Assert.Throws<ArgumentException>("text", () => CrossCompanyCorrelatingId.Compute(""));
+    }
+
+    [Fact]
+    public void C3Id_EmptyRaw_Throws()
+    {
+        byte[] destination = new byte[CrossCompanyCorrelatingId.RawSizeInBytes];
+        Assert.Throws<ArgumentException>("text", () => CrossCompanyCorrelatingId.ComputeRaw("", destination));
+    }
+
+    [Fact]
+    public void C3Id_EmptyRawSpan_Throws()
+    {
+        byte[] destination = new byte[CrossCompanyCorrelatingId.RawSizeInBytes];
+        Assert.Throws<ArgumentException>("text", () => CrossCompanyCorrelatingId.ComputeRaw([], destination));
+    }
+
+    [Fact]
+    public void C3Id_EmptyRawUtf8_Throws()
+    {
+        byte[] destination = new byte[CrossCompanyCorrelatingId.RawSizeInBytes];
+        Assert.Throws<ArgumentException>("textUtf8", () => CrossCompanyCorrelatingId.ComputeRawUtf8([], destination));
+    }
+
+    [Fact]
+    public void C3Id_DestinationTooSmall_Throws()
+    {
+        byte[] destination = new byte[CrossCompanyCorrelatingId.RawSizeInBytes - 1];
+        Assert.Throws<ArgumentException>(
+            "destination",
+            () => CrossCompanyCorrelatingId.ComputeRaw("test", destination));
+    }
+
+    [Fact]
+    public void C3Id_DestinationTooSmallUtf8_Throws()
+    {
+        byte[] destination = new byte[CrossCompanyCorrelatingId.RawSizeInBytes - 1];
+        Assert.Throws<ArgumentException>(
+            "destination",
+            () => CrossCompanyCorrelatingId.ComputeRawUtf8("test"u8, destination));
+    }
+
+    private static string ComputeC3Id(string text)
+    {
+        string reference = ReferenceCrossCompanyCorrelatingId.Compute(text);
+        string actual = CrossCompanyCorrelatingId.Compute(text);
+
+        Assert.True(
+            actual == reference,
+            $"""
+            Actual implementation did not match reference implementation for '{text}'.
+
+              reference: {reference}
+                 actual: {actual}
+            """);
+
+        return actual;
+    }
+
+    /// <summary>
+    /// A trivial reference implementation of C3ID that is easy to understand,
+    /// but not optimized for performance. We compare this to the production
+    /// implementation to ensure that it remains equivalent to this.
+    /// </summary>
+    private static class ReferenceCrossCompanyCorrelatingId
+    {
+        public static string Compute(string text)
+        {
+            // Compute the SHA-256 hash of the UTF8-encoded text
+            Span<byte> hash = SHA256.HashData(Encoding.UTF8.GetBytes(text));
+
+            // Prefix the result with "C3ID" UTF-8 bytes and hash again
+            hash = SHA256.HashData([.. "C3ID"u8, .. hash]);
+
+            // Truncate to 12 bytes
+            hash = hash[..12];
+
+            // Convert to base64 and prepend "C3ID"
+            return "C3ID" + Convert.ToBase64String(hash);
+        }
+    }
+}
diff --git a/src/Tests/Cask.Tests/PolyfillTests.cs b/src/Tests/Cask.Tests/PolyfillTests.cs
index 232e5f9..be379d1 100644
--- a/src/Tests/Cask.Tests/PolyfillTests.cs
+++ b/src/Tests/Cask.Tests/PolyfillTests.cs
@@ -271,6 +271,31 @@ public void Random_NotDeterministic()
         Assert.False(random1.SequenceEqual(random2), "RandomNumberGenerator produced two identical 32-byte sequences.");
     }
 
+    [Fact]
+    public void Encoding_GetString_Empty()
+    {
+        ReadOnlySpan<byte> data = [];
+        string text = Encoding.UTF8.GetString(data);
+        Assert.Equal("", text);
+    }
+
+    [Fact]
+    public void Encoding_GetByteCount_Empty()
+    {
+        ReadOnlySpan<char> text = "".AsSpan();
+        int byteCount = Encoding.UTF8.GetByteCount(text);
+        Assert.Equal(0, byteCount);
+    }
+
+    [Fact]
+    public void Encoding_GetBytes_Empty()
+    {
+        ReadOnlySpan<char> text = "".AsSpan();
+        Span<byte> bytes = [];
+        int bytesWritten = Encoding.UTF8.GetBytes(text, bytes);
+        Assert.Equal(0, bytesWritten);
+    }
+
 #if NETFRAMEWORK // We don't need to stress test the modern BCL :)
     [Fact]
     public async Task Polyfill_ThreadingStress()