diff --git a/protocol/osrs-221-model/src/main/kotlin/net/rsprot/protocol/loginprot/incoming/util/CyclicRedundancyCheckBlock.kt b/protocol/osrs-221-model/src/main/kotlin/net/rsprot/protocol/loginprot/incoming/util/CyclicRedundancyCheckBlock.kt
new file mode 100644
index 000000000..9b0a3e4d1
--- /dev/null
+++ b/protocol/osrs-221-model/src/main/kotlin/net/rsprot/protocol/loginprot/incoming/util/CyclicRedundancyCheckBlock.kt
@@ -0,0 +1,31 @@
+package net.rsprot.protocol.loginprot.incoming.util
+
+/**
+ * CRC blocks are helper structures used for the server to verify that the CRC is up-to-date.
+ * As the client transmits less CRCs than there are cache indices, we provide validation methods
+ * through this abstract class at the respective revision's decoder level, so we can perform checks
+ * that correspond to the information received from the client, and not what the server fully knows of.
+ * @property clientCrc the int array of client CRCs, indexed by the cache archives.
+ */
+public abstract class CyclicRedundancyCheckBlock(
+    protected val clientCrc: IntArray,
+) {
+    public abstract fun validate(serverCrc: IntArray): Boolean
+
+    override fun equals(other: Any?): Boolean {
+        if (this === other) return true
+        if (other !is CyclicRedundancyCheckBlock) return false
+
+        if (!clientCrc.contentEquals(other.clientCrc)) return false
+
+        return true
+    }
+
+    override fun hashCode(): Int {
+        return clientCrc.contentHashCode()
+    }
+
+    override fun toString(): String {
+        return "CyclicRedundancyCheckBlock(clientCrc=${clientCrc.contentToString()})"
+    }
+}
diff --git a/protocol/osrs-221-model/src/main/kotlin/net/rsprot/protocol/loginprot/incoming/util/LoginBlock.kt b/protocol/osrs-221-model/src/main/kotlin/net/rsprot/protocol/loginprot/incoming/util/LoginBlock.kt
index 3f80ddbd7..1ae3bf395 100644
--- a/protocol/osrs-221-model/src/main/kotlin/net/rsprot/protocol/loginprot/incoming/util/LoginBlock.kt
+++ b/protocol/osrs-221-model/src/main/kotlin/net/rsprot/protocol/loginprot/incoming/util/LoginBlock.kt
@@ -23,7 +23,7 @@ public class LoginBlock<T>(
     public val hostPlatformStats: HostPlatformStats,
     private val _secondClientType: UByte,
     private val _crcBlockHeader: UByte,
-    public val crc: IntArray,
+    public val crc: CyclicRedundancyCheckBlock,
     public val authentication: T,
 ) : IncomingMessage {
     public val firstClientType: Int
@@ -68,7 +68,7 @@ public class LoginBlock<T>(
         if (hostPlatformStats != other.hostPlatformStats) return false
         if (_secondClientType != other._secondClientType) return false
         if (_crcBlockHeader != other._crcBlockHeader) return false
-        if (!crc.contentEquals(other.crc)) return false
+        if (crc != other.crc) return false
         if (authentication != other.authentication) return false
 
         return true
@@ -94,7 +94,7 @@ public class LoginBlock<T>(
         result = 31 * result + hostPlatformStats.hashCode()
         result = 31 * result + _secondClientType.hashCode()
         result = 31 * result + _crcBlockHeader.hashCode()
-        result = 31 * result + crc.contentHashCode()
+        result = 31 * result + crc.hashCode()
         result = 31 * result + authentication.hashCode()
         return result
     }
@@ -112,7 +112,7 @@ public class LoginBlock<T>(
             "siteSettings='$siteSettings', " +
             "affiliate=$affiliate, " +
             "hostPlatformStats=$hostPlatformStats, " +
-            "crc=${crc.contentToString()}, " +
+            "crc=$crc, " +
             "firstClientType=$firstClientType, " +
             "platformType=$platformType, " +
             "constZero1=$constZero1, " +
diff --git a/protocol/osrs-221-shared/src/main/kotlin/net/rsprot/protocol/common/loginprot/incoming/codec/shared/LoginBlockDecoder.kt b/protocol/osrs-221-shared/src/main/kotlin/net/rsprot/protocol/common/loginprot/incoming/codec/shared/LoginBlockDecoder.kt
index 64efbf231..e3ea79260 100644
--- a/protocol/osrs-221-shared/src/main/kotlin/net/rsprot/protocol/common/loginprot/incoming/codec/shared/LoginBlockDecoder.kt
+++ b/protocol/osrs-221-shared/src/main/kotlin/net/rsprot/protocol/common/loginprot/incoming/codec/shared/LoginBlockDecoder.kt
@@ -3,6 +3,7 @@ package net.rsprot.protocol.common.loginprot.incoming.codec.shared
 import net.rsprot.buffer.JagByteBuf
 import net.rsprot.protocol.cryptography.decipherRsa
 import net.rsprot.protocol.cryptography.xteaDecrypt
+import net.rsprot.protocol.loginprot.incoming.util.CyclicRedundancyCheckBlock
 import net.rsprot.protocol.loginprot.incoming.util.HostPlatformStats
 import net.rsprot.protocol.loginprot.incoming.util.LoginBlock
 import java.math.BigInteger
@@ -52,11 +53,7 @@ public abstract class LoginBlockDecoder<T>(
         val hostPlatformStats = decodeHostPlatformStats(xteaBuffer)
         val secondClientType = xteaBuffer.g1()
         val crcBlockHeader = xteaBuffer.g4()
-        // As revision 221 isn't out yet, we will just naively read the values
-        val crc =
-            IntArray(21) {
-                xteaBuffer.g4()
-            }
+        val crc = decodeCrc(xteaBuffer)
         return LoginBlock(
             version,
             subVersion,
@@ -82,6 +79,46 @@ public abstract class LoginBlockDecoder<T>(
         )
     }
 
+    private fun decodeCrc(buffer: JagByteBuf): CyclicRedundancyCheckBlock {
+        val transmittedCount = 21
+        val crc = IntArray(transmittedCount)
+        crc[0] = buffer.g4Alt1()
+        crc[4] = buffer.g4Alt3()
+        crc[15] = buffer.g4Alt3()
+        crc[17] = buffer.g4Alt1()
+        crc[13] = buffer.g4Alt2()
+        crc[16] = buffer.g4Alt1()
+        crc[1] = buffer.g4Alt1()
+        crc[10] = buffer.g4()
+        crc[9] = buffer.g4Alt2()
+        crc[7] = buffer.g4()
+        crc[8] = buffer.g4Alt2()
+        crc[6] = buffer.g4Alt1()
+        crc[20] = buffer.g4Alt3()
+        crc[19] = buffer.g4Alt3()
+        crc[14] = buffer.g4()
+        crc[12] = buffer.g4()
+        crc[5] = buffer.g4Alt2()
+        crc[3] = buffer.g4Alt3()
+        crc[11] = buffer.g4Alt3()
+        crc[2] = buffer.g4Alt3()
+        crc[18] = buffer.g4Alt3()
+
+        return object : CyclicRedundancyCheckBlock(crc) {
+            override fun validate(serverCrc: IntArray): Boolean {
+                require(serverCrc.size >= transmittedCount) {
+                    "Server CRC length less than expected: ${serverCrc.size}, expected >= $transmittedCount"
+                }
+                for (i in 0..<transmittedCount) {
+                    if (serverCrc[i] != this.clientCrc[i]) {
+                        return false
+                    }
+                }
+                return true
+            }
+        }
+    }
+
     private fun decodeHostPlatformStats(buffer: JagByteBuf): HostPlatformStats {
         val version = buffer.g1()
         val osType = buffer.g1()